Effects of DNA intercalating agents on topoisomerase II induced DNA strand cleavage in isolated mammalian cell nuclei

Synthesis, characterization, interactions with the DNA duplex dodecamer d(5'-CGCGAATTCGCG-3')2 and cytotoxicity of binuclear η6-arene-Ru(II) complexes

The novel binuclear η⁶-arene-Ru(II) complexes with the general formula {[(η⁶-cym)Ru(L)]₂(μ-BL)}(PF₆)₄, and their corresponding water soluble {[(η⁶-cym)Ru(L)]₂(μ-BL)}Cl₄, where cym = p-cymene, L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen), BL = 4,4'-bipyridine (BL-1), 1,2-bis(4-pyridyl)ethane (BL-2) and 1,3-bis(4-pyridyl)propane (BL-3), were synthesized and characterized. The structure of {[(η⁶-cym)Ru(phen)]₂(μ-BL-1)}(PF₆)₄ was determined by X-ray single crystal methods. The interaction of {[(η⁶-cym)Ru(phen)]₂(μ-BL-i)}Cl₄ (i = 1, 2, 3; (4), (5) and (6) correspondingly) with the DNA duplex d(5'-CGCGAATTCGCG-3')₂ was studied by means of NMR techniques and fluorescence titrations. The results show that complex (4) binds with a K_b = 12.133 × 10³ M^-1 through both intercalation and groove binding, while (5) and (6) are groove binders (K_b = 2.333 × 10³ M^-1 and K_b = 3.336 × 10³ M^-1 correspondingly). Comparison with the mononuclear complex [(η⁶-cym)Ru(phen)(py)]²⁺ reveals that it binds to the d(5'-CGCGAATTCGCG-3')₂ with a K_b value two orders of magnitude lower than (4) (K_b = 0.158 × 10³ M^-1), indicating that for the binuclear complexes both ruthenium moieties participate in the binding. The complexes were found to be cytotoxic against the A2780 and A2780 res. cancer cell line with a selectivity index (SI) in the range of 3.0-5.9.

The Critical Role of 12-Methyl Group of Anthracycline Dutomycin to Its Antiproliferative Activity

Anthracycline dutomycin is a tetracyclic quinone glycoside produced by Streptomyces minoensis NRRL B-5482. SW91 is a C-12 demethylated dutomycin derivative, which was identified in our previous research. In vitro cytotoxicity and apoptosis assays of these two compounds were conducted to demonstrate their antiproliferation activities. The results showed that both dutomycin and SW91 block cells at the S phase, whereas dutomycin shows more significant inhibition of cell growth. Their interactions with calf thymus DNA (CT-DNA) were investigated, with dutomycin exhibiting higher binding affinity. The molecular docking demonstrated that the 12-methyl group makes dutomycin attach to the groove of DNA. These findings suggest that dutomycin has binding higher affinity to DNA and impairs DNA replication resulting in more significant antitumor activity.

Natural Compounds as Therapeutic Agents: The Case of Human Topoisomerase IB

Natural products are widely used as source for drugs development. An interesting example is represented by natural drugs developed against human topoisomerase IB, a ubiquitous enzyme involved in many cellular processes where several topological problems occur due the formation of supercoiled DNA. Human topoisomerase IB, involved in the solution of such problems relaxing the DNA cleaving and religating a single DNA strand, represents an important target in anticancer therapy. Several natural compounds inhibiting or poisoning this enzyme are under investigation as possible new drugs. This review summarizes the natural products that target human topoisomerase IB that may be used as the lead compounds to develop new anticancer drugs. Moreover, the natural compounds and their derivatives that are in clinical trial are also commented on.

New insights into the activities and toxicities of the old anticancer drug doxorubicin

The anthracycline drug doxorubicin is among the most used—and useful—chemotherapeutics. While doxorubicin is highly effective in the treatment of various hematopoietic malignancies and solid tumours, its application is limited by severe adverse effects, including irreversible cardiotoxicity, therapy‐related malignancies and gonadotoxicity. This continues to motivate investigation into the mechanisms of anthracycline activities and toxicities, with the aim to overcome the latter without sacrificing the former. It has long been appreciated that doxorubicin causes DNA double‐strand breaks due to poisoning topoisomerase II. More recently, it became clear that doxorubicin also leads to chromatin damage achieved through eviction of histones from select sites in the genome. Evaluation of these activities in various anthracycline analogues has revealed that chromatin damage makes a major contribution to the efficacy of anthracycline drugs. Furthermore, the DNA‐damaging effect conspires with chromatin damage to cause a number of adverse effects. Structure–activity relationships within the anthracycline family offer opportunities for chemical separation of these activities towards development of effective analogues with limited adverse effects. In this review, we elaborate on our current understanding of the different activities of doxorubicin and their contributions to drug efficacy and side effects. We then offer our perspective on how the activities of this old anticancer drug can be amended in new ways to benefit cancer patients, by providing effective treatment with improved quality of life.

Chronophin activation is necessary in Doxorubicin-induced actin cytoskeleton alteration

Although doxorubicin (Dox)-induced oxidative stress is known to be associated with cytotoxicity, the precise mechanism remains unclear. Genotoxic stress not only generates free radicals, but also affects actin cytoskeleton stability. We showed that Dox-induced RhoA signaling stimulated actin cytoskeleton alterations, resulting in central stress fiber disruption at early time points and cell periphery cortical actin formation at a later stage, in HeLa cells. Interestingly, activation of a cofilin phosphatase, chronophin (CIN), was initially evoked by Dox-induced RhoA signaling, resulting in a rapid phosphorylated cofilin turnover leading to actin cytoskeleton remodeling. In addition, a novel interaction between CIN and 14-3-3ζ was detected in the absence of Dox treatment. We demonstrated that CIN activity is quite contrary to 14-3-3ζ binding, and the interaction leads to enhanced phosphorylated cofilin levels. Therefore, initial CIN activation regulation could be critical in Dox-induced actin cytoskeleton remodeling through RhoA/cofilin signaling. [BMB Reports 2017; 50(6): 335-340].

pH-responsive mesoporous ZSM-5 zeolites/chitosan core-shell nanodisks loaded with doxorubicin against osteosarcoma

Oral or intravenous chemotherapy is an important strategy to treat metastatic cancer, but it may cause systemic toxicity for healthy tissue. Herein, we for the first time fabricated mesoporous ZSM-5 zeolites/chitosan core-shell nanodisks loaded with doxorubicin (ZSM-5/CS/DOX) as drug delivery systems against osteosarcoma. The mesoporous ZSM-5 zeolites exhibited disk-like shapes with thicknesses of 100nm and diameters of 300nm, and the mesopores with pore sizes of 3.75nm were originated from desilication treatment. The pH-responsive ZSM-5/CS/DOX nanodisks possessed a great drug loading efficiency of 97.7%, and their controlled release trends of DOX were fitted well with the Korsmeyer-Peppas model. The DOX could be efficiently released the ZSM-5/CS/DOX nanodisks after cellular endocytosis and induced cancer cells apoptosis. Moreover, the pH-responsive drug carriers led to efficient tumor inhibition with low side effects, especially cardiac toxicity, as confirmed by pharmacokinetic study, serological examination and H&E staining assays. Therefore, the ZSM-5/CS/DOX nanodisks are a promising pH-responsive drug carrier for targeted cancer therapy.

Insight into the Mechanism of Action of Marine Cytotoxic Thiazinoquinones

The electrochemical response of four natural cytotoxic thiazinoquinones isolated from the Aplidium species was studied using conventional solution-phase and solid-state techniques, based on the voltammetry of immobilized particles methodology. The interaction with O₂ and electrochemically generated reactive oxygen species (ROS) was electrochemically monitored. At the same time, a molecular modeling study including density functional theory (DFT) calculations was performed in order to analyze the conformational and electronic properties of the natural thiazinoquinones, as well as those of their reduced intermediates. The obtained electrochemical and computational results were analyzed and correlated to cytotoxic activity of these compounds, highlighting some features possibly related to their mechanism of action.

Hollow mesoporous ZSM-5 zeolite/chitosan ellipsoids loaded with doxorubicin as pH-responsive drug delivery systems against osteosarcoma

Hollow mesoporous ZSM-5 zeolite/chitosan ellipsoids loaded with doxorubicin were firstly reported as novel pH-responsive drug delivery systems, and the controlled release of doxorubicin effectively treated osteosarcoma without systemic toxicity.

Mechanisms of small molecule-DNA interactions probed by single-molecule force spectroscopy

There is a wide range of applications for non-covalent DNA binding ligands, and optimization of such interactions requires detailed understanding of the binding mechanisms. One important class of these ligands is that of intercalators, which bind DNA by inserting aromatic moieties between adjacent DNA base pairs. Characterizing the dynamic and equilibrium aspects of DNA-intercalator complex assembly may allow optimization of DNA binding for specific functions. Single-molecule force spectroscopy studies have recently revealed new details about the molecular mechanisms governing DNA intercalation. These studies can provide the binding kinetics and affinity as well as determining the magnitude of the double helix structural deformations during the dynamic assembly of DNA–ligand complexes. These results may in turn guide the rational design of intercalators synthesized for DNA-targeted drugs, optical probes, or integrated biological self-assembly processes. Herein, we survey the progress in experimental methods as well as the corresponding analysis framework for understanding single molecule DNA binding mechanisms. We discuss briefly minor and major groove binding ligands, and then focus on intercalators, which have been probed extensively with these methods. Conventional mono-intercalators and bis-intercalators are discussed, followed by unconventional DNA intercalation. We then consider the prospects for using these methods in optimizing conventional and unconventional DNA-intercalating small molecules.

Evaluation of the mutagenicity and antimutagenicity of Ziziphus joazeiro Mart. bark in the micronucleus assay

The aim of this study was to evaluate the mutagenicity (clastogenicity/aneugenicity) of a glycolic extract of Ziziphus joazeiro bark (GEZJ) by the micronucleus assay in mice bone marrow. Antimutagenic activity was also assessed using treatments associated with GEZJ and doxorubicin (DXR). Mice were evaluated 24–48 h after exposure to positive (N-nitroso-N-ethylurea, NEU - 50 mg.kg⁻¹ and DXR - 5 mg.kg⁻¹) and negative (150 mM NaCl) controls, as well as treatment with GEZJ (0.5–2 g.kg⁻¹), GEZJ (2 g.kg⁻¹) + NEU and GEZJ (2 g.kg⁻¹) + DXR. There were no significant differences in the frequencies of micronucleated polychromatic erythrocytes in mice treated with GEJZ and GEJZ + DXR compared to the negative controls, indicating that GEZJ was not mutagenic. Analysis of the polychromatic:normochromatic erythrocyte ratio revealed significant differences in the responses to doses of 0.5 g.kg⁻¹ and 1–2 g.kg⁻¹ and the positive control (NEU). These results indicated no systemic toxicity and moderate toxicity at lower and higher doses of GEZJ. The lack of mutagenicity and systemic toxicity in the antimutagenic assays, especially for treatment with GEZJ + DXR, suggested that phytochemical compounds in Z. joazeiro bark attenuated DXR-induced mutagenicity and the moderate systemic toxicity of a high dose of Z. joazeiro bark (2 g.kg⁻¹). Further studies on the genotoxicity of Z. joazeiro extracts are necessary to establish the possible health risk in humans and to determine the potential as a chemopreventive agent for therapeutic use.

Time course changes of anti- and pro-apoptotic proteins in apigenin-induced genotoxicity

Background

Apigenin (4′,5,7-trihydroxyflavone, AP), an active component of many medicinal Chinese herbs, exhibits anticancer properties in vitro and in vivo. This study aims to investigate the genotoxic, cytostatic, and cytotoxic effects of AP and time course changes in the levels of anti- and pro-apoptotic proteins involved in the DNA damage response in HepG2 cells.

Methods

The genotoxic potential of AP was determined by sister chromatid exchanges (SCEs) and chromosomal aberrations (CAs) analysis. The levels of cytostaticity and cytotoxicity were evaluated by the proliferation rate and mitotic indices, respectively. MTT was used to study cytotoxicity, while the induction of apoptosis and the expression of apoptosis-related proteins were determined by ELISA.

Results

At concentrations greater than 10 μM, AP decreased cell survival in a dose- (48 h: 10 vs. 20 μΜ, P < 0.001 and 20 vs. 50 μΜ, P = 0.005; 72 h: 10 vs. 20 μΜ, P < 0.001 and 20 vs. 50 μΜ, P = 0.001) and time-dependent manner (20 μΜ: 24 vs. 48 h, P < 0.001 and 48 vs. 72 h, P = 0.003; 50 μΜ: 24 vs. 48 h, P < 0.001 and 48 vs. 72 h, P < 0.001; 100 μΜ: 24 vs. 48 h, P < 0.001 and 48 vs. 72 h, P < 0.001). SCEs rates, cell proliferation, and mitotic divisions were also affected in a dose-dependent manner (P < 0.001). There was no change in the frequency of aberrant cells (1 μΜ ΑP: P = 0.554; 10 μM AP: P = 0.337; 20 μΜ AP: P = 0.239). Bcl-2 levels were reduced 3 h after AP administration (P = 0.003) and remained reduced throughout the 48 h observation period (6 h, P = 0.044; 12 h, P = 0.001; 24 h, P = 0.042; 48 h, P = 0.012). Bax and soluble Fas exhibited a transient upregulation 24 h after AP treatment. The Bax/Bcl-2 ratio was also increased at 12 h and remained increased throughout the 48 h observation period.

Conclusion

AP exhibited dose-dependent genotoxic potential in HepG2 cells. The protein levels of sFas, Bcl-2, and Bax were affected by AP to promote cell survival and cell death, respectively.

Identification of G-Quadruplex Inducers Usinga Simple, Inexpensiveand Rapid High Throughput Assay, and TheirInhibition of Human Telomerase

Telomeres are protein and DNA complexes located atchromosome ends. Telomeric DNA is composed of a double stranded region of repetitive DNA followed by single-stranded 3' extension of aG-rich sequence. Single-stranded G-rich sequencescan fold into G-quadruplex structures,and molecules that stabilize G-quadruplexes are known to inhibit the enzyme telomerase and disrupt telomere maintenance. Because telomere maintenance is required for proliferation of cancer cells, G-quadruplex stabilizers have become attractive prospects for anticancer drug discovery.However, telomere-targeting G-quadruplex ligands have yet to enter the clinic owing in part to poor pharmacokinetics and target selectivity. Increasing the pharmacophore diversity of G-quadruplex and specifically telomeric-DNA targeting agents should assist in overcoming these shortcomings. In this work, we report the identification and validation ofligands that bind telomeric DNA and induce G-quadruplex formationusing the NCI Diversity Set I, providing validation of anextremely simple, rapid and high-throughput screen using FRET technology. Hits from the screen were validated by examining telomerase inhibition and G-quadruplex inductionusing CD spectroscopy and DNA polymerase stop assays. We show that two known DNA binding molecules, ellipticine derivativeNSC 176327 (apyridocarbazole) and NSC 305831 (an antiparasitic hetero-cyclediamidine referred to as furamidine and DB75),are selective induceG-quadruplex formation in the human telomeric sequence and bind telomeric DNA quadruplexes in the absence of stabilizing monovalent cations with molar ratios(molecule: DNA)of 4:1and 1.5:1, respectively.

Evodiamine, a dual catalytic inhibitor of type I and II topoisomerases, exhibits enhanced inhibition against camptothecin resistant cells

DNA topoisomerases are nuclear enzymes that are the targets for several anticancer drugs. In this study we investigated the antiproliferative activity against human leukaemia cell lines and the effects on topoisomerase I and II of evodiamine, which is a quinazolinocarboline alkaloid isolated from the fruit of a traditional Chinese medicinal plant, Evodia rutaecarpa. We report here the anti-proliferative activity against human leukaemia cells K562, THP-1, CCRF-CEM and CCRF-CEM/C1 and the inhibitory mechanism on human topoisomerases I and II, important anti-cancer drugs targets, of evodiamine. Evodiamine failed to trap [Topo-DNA] complexes and induce any detectable DNA damage in cells, was unable to bind or intercalate DNA, and arrested cells in the G(2)/M phase. The results suggest evodiamine is a dual catalytic inhibitor of topoisomerases I and II, with IC(50) of 60.74 and 78.81 μM, respectively. The improved toxicity towards camptothecin resistant cells further supports its inhibitory mechanism which is different from camptothecin, and its therapeutic potential.

Spectroscopic investigations of the molecular interaction of anticancer drug mitoxantrone with non-ionic surfactant micelles

Objectives

The aim of this study was to investigate the interaction of the anticancer drug mitoxantrone with non-ionic micelles, as simple model systems of biological membranes.

Methods

UV-VIS absorption spectroscopy was used to quantify the drug–surfactant micelle interactions in terms of the binding constant and the micelle–water partition coefficient of the drug.

Key findings

Interaction of mitoxantrone with non-ionic micelles reduces the dimerization process of mitoxantrone, the drug molecules being encapsulated into micelles as monomer. The strength of the interaction between mitoxantrone and non-ionic micelles is higher at pH 10 than at pH 7.4, and depends on the surfactant in the order Tween 80 &gt; Tween 20 &gt; Triton X-100. The higher partition coefficient at pH 10 compared to pH 7.4 suggests that at basic pH the deprotonated mitoxantrone is incorporated more efficiently into the hydrophobic medium of non-ionic micelles compared to physiological pH, when the protonated drug is predominant.

Conclusions

These results on simple model systems miming the drug–membrane interactions contribute to the elucidation of the behaviour of the drug in vivo, as well as the possible utilization of surfactant micelles as drug carriers.

Involvement of nucleotide excision and mismatch repair mechanisms in double strand break repair

Living organisms are constantly threatened by environmental DNA-damaging agents, including UV and ionizing radiation (IR). Repair of various forms of DNA damage caused by IR is normally thought to follow lesion-specific repair pathways with distinct enzymatic machinery. DNA double strand break is one of the most serious kinds of damage induced by IR, which is repaired through double strand break (DSB) repair mechanisms, including homologous recombination (HR) and non-homologous end joining (NHEJ). However, recent studies have presented increasing evidence that various DNA repair pathways are not separated, but well interlinked. It has been suggested that non-DSB repair mechanisms, such as Nucleotide Excision Repair (NER), Mismatch Repair (MMR) and cell cycle regulation, are highly involved in DSB repairs. These findings revealed previously unrecognized roles of various non-DSB repair genes and indicated that a successful DSB repair requires both DSB repair mechanisms and non-DSB repair systems. One of our recent studies found that suppressed expression of non-DSB repair genes, such as XPA, RPA and MLH1, influenced the yield of IR induced micronuclei formation and/or chromosome aberrations, suggesting that these genes are highly involved in DSB repair and DSB-related cell cycle arrest, which reveals new roles for these gene products in the DNA repair network. In this review, we summarize current progress on the function of non-DSB repair-related proteins, especially those that participate in NER and MMR pathways, and their influence on DSB repair. In addition, we present our developing view that the DSB repair mechanisms are more complex and are regulated by not only the well known HR/NHEJ pathways, but also a systematically coordinated cellular network.

Development of a novel assay for human tyrosyl DNA phosphodiesterase 2

Tyrosyl DNA phosphodiesterase 2 (TDP2), a newly discovered enzyme that cleaves 5'-phosphotyrosyl bonds, is a potential target for chemotherapy. TDP2 possesses both 3'- and 5'-tyrosyl-DNA phosphodiesterase activity, which is generally measured in a gel-based assay using 3'- and 5'-phosphotyrosyl linkage at the 3' and 5' ends of an oligonucleotide. To understand the enzymatic mechanism of this novel enzyme, the gel-based assay is useful, but this technique is cumbersome for TDP2 inhibitor screening. For this reason, we have designed a novel assay using p-nitrophenyl-thymidine-5'-phosphate (T5PNP) as a substrate. This assay can be used in continuous colorimetric assays in a 96-well format. We compared the salt and pH effect on product formation with the colorimetric and gel-based assays and showed that they behave similarly. Steady-state kinetic studies showed that the 5' activity of TDP2 is 1000-fold more efficient than T5PNP. Tyrosyl DNA phosphodiesterase 1 (TDP1) and human AP-endonuclease 1 (APE1) could not hydrolyze T5PNP. Sodium orthovanadate, a known inhibitor of TDP2, inhibits product formation from T5PNP by TDP2 (IC(50)=40 mM). Our results suggest that this novel assay system with this new TDP2 substrate can be used for inhibitor screening in a high-throughput manner.

Newly identified antibacterial compounds are topoisomerase poisons in African trypanosomes

Human African trypanosomiasis, caused by the Trypanosoma brucei protozoan parasite, is fatal when left untreated. Current therapies are antiquated, and there is a need for new pharmacologic agents against T. brucei targets that have no human ortholog. Trypanosomes have a single mitochondrion with a unique mitochondrial DNA, known as kinetoplast DNA (kDNA), a topologically complex network that contains thousands of interlocking circular DNAs, termed minicircles (approximately 1 kb) and maxicircles (approximately 23 kb). Replication of kDNA depends on topoisomerases, enzymes that catalyze reactions that change DNA topology. T. brucei has an unusual type IA topoisomerase that is dedicated to kDNA metabolism. This enzyme has no ortholog in humans, and RNA interference (RNAi) studies have shown that it is essential for parasite survival, making it an ideal drug target. In a large chemical library screen, two compounds were recently identified as poisons of bacterial topoisomerase IA. We found that these compounds are trypanocidal in the low micromolar range and that they promote the formation of linearized minicircles covalently bound to protein on the 5' end, consistent with the poisoning of mitochondrial topoisomerase IA. Surprisingly, however, band depletion studies showed that it is topoisomerase IImt, and not topoisomerase IAmt, that is trapped. Both compounds are planar aromatic polycyclic structures that intercalate into and unwind DNA. These findings reinforce the utility of topoisomerase IImt as a target for development of new drugs for African sleeping sickness.

Antiproliferative and antiangiogenic effects of the benzophenanthridine alkaloid sanguinarine in melanoma

This study was aimed at evaluating the potential application of benzophenanthridine alkaloids, sanguinarine and cheleritrine, in the therapy of melanoma cancer. In vitro antiproliferative activity of sanguinarine was higher than that of cheleritrine against the B16 melanoma 4A5 cells. Both agents were able to produce DNA breaks, and the DNA unwinding assay showed that they act as DNA intercalating agents. Sanguinarine was selected for determination of its in vivo preclinical efficacy. Oral treatment with sanguinarine reduced the tumor burden in a transplantable murine tumor grown in a syngeneic host (B16 melanoma 4A5 in C57BL/6 mice), and in a human tumor xenograft grown in immunodeficient mice (A375 human melanoma in athymic nude mice). In A375 tumors a significant decrease in the proliferation marker Ki67, and a reduction in the activated mitogen-activated protein kinases (p-p44/42 MAPK), and in protein kinase B (pAKT) were also observed. Three out of eleven A375-bearing treated mice were tumor-free at the end of treatment, and did not develop any tumor after a further, treatment-free, observation period of 60 days. Sanguinarine also showed a striking antiangiogenic activity in mice. Data from the present study support the concept that sanguinarine can be effective in melanoma skin cancer.

Targeting DNA topoisomerase II in cancer chemotherapy

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

Isolation and characterization of mAMSA-hypersensitive mutants. Cytotoxicity of Top2 covalent complexes containing DNA single strand breaks

Topoisomerase II (Top2) is the primary target for active anti-cancer agents. We developed an efficient approach for identifying hypersensitive Top2 mutants and isolated a panel of mutants in yeast Top2 conferring hypersensitivity to the intercalator N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulphonanilide (mAMSA). Some mutants conferred hypersensitivity to etoposide as well as mAMSA, whereas other mutants exhibited hypersensitivity only to mAMSA. Two mutants in Top2, changing Pro(473) to Leu and Gly(737) to Val, conferred extraordinary hypersensitivity to mAMSA and were chosen for further characterization. The mutant proteins were purified, and their biochemical activities were assessed. Both mutants encode enzymes that are hypersensitive to inhibition by mAMSA and other intercalating agents and exhibited elevated levels of mAMSA-induced Top2:DNA covalent complexes. While Gly(737) --> Val Top2p generated elevated levels of Top2-mediated double strand breaks in vitro, the Pro(473) --> Leu mutant protein showed only a modest increase in Top2-mediated double strand breaks but much higher levels of Top2-mediated single strand breaks. In addition, the Pro(473) --> Leu mutant protein also generated high levels of mAMSA-stabilized covalent complexes in the absence of ATP. We tested the role of single strand cleavage in cell killing with alleles of Top2 that could generate single strand breaks, but not double strand breaks. Expression in yeast of a Pro(473) --> Leu mutant that could only generate single strand breaks conferred hypersensitivity to mAMSA. These results indicate that generation of single strand breaks by Top2-targeting agents can be an important component of cell killing by Top2-targeting drugs.

Protection of multiple antioxidants Chinese herbal medicine on the oxidative stress induced by adriamycin chemotherapy

Adriamycin is an effective anthracycline anti-tumor antibiotic. However, the clinical use of adriamycin has been restricted by its serious side effects. Some reports indicated that the side effects of adriamycin could cause systemic injury, in which reactive oxygen species (ROS) play an important role. ROS are a large family of oxygen free radical and non-free radical active oxygen-containing molecules, including superoxide radical, hydrogen peroxide and hydroxyl radical, which contribute to oxidative stress. Although antioxidant treatment is a promising method to prevent the side effects, protection by a single antioxidant is limited. The Chinese herbal medicine ANTIOXIN is a multiple antioxidant that can effectively block oxidative stress. It was hypothesized that ANTIOXIN could effectively reduce the side effects of adriamycin. A rat tumor model with a transplanted tumor in the liver was treated with adriamycin and ANTIOXIN was used as a protection. Oxidative stress and antioxidant enzymes were evaluated. The results showed that adriamycin chemotherapy increased the level of malondialdehyde (MDA), nitrogen oxide (NO) and decreased the activities of total superoxide dismutase (T-SOD), manganese superoxide dismutase (MnSOD), catalase (CAT), glutathione (GSH) and total antioxidant capacity (TAC). Adriamycin chemotherapy also decreased the expression of Bcl-2, increased the expression of iNOS and cell apoptosis in the liver and kidney. Multiple antioxidants ANTIOXIN had an antagonistic effect on these changes and significantly decreased the mortality of the experimental rats. These data demonstrated that adriamycin chemotherapy could cause oxidative stress to the whole body, on which multiple antioxidants based on the theory of 'multiple antioxidant chain' had effective protection.

Anthracycline-based combined chemotherapy in the mouse model

Our research was aimed at establishing if and how selenium (Se) ion, N-acetylcysteine (NAC), sodium salt of monoketocholic acid (MKH) and superoxide-dismutase (SOD), administered in the experimental animal model, could affect the possible cytotoxicity associated with anthracycline-based combined chemotherapy with doxorubicin, vincristine and prednisolone (DVP). The following biochemical parameters were investigated: the extent of lipid peroxidation (LPx), and the activity of peroxidase (Px), catalase (CAT), glutathione-peroxidase (GSHPx), and xanthine-oxidase (XOD). A statistical increase in LPx activity was obtained by SOD, MKH, DVPSe and DVPMKH. All chemotherapeutic agents reduced Px activity in a statistically significant manner. There was no statistical significance for the results regarding the effects of the administered substances on GSHPx activity. The results for DVP, SOD, MKH, DVPSOD, DVPSe and DVPMKH showed reduced XOD activity which was statistically significant, which was lowest in the case of MKH, while NAC and Se reduced the activity of this enzyme but statistically non significant. NAC, Se, DVP, MKH and DVPMKH caused a reduction in CAT activity, while DVPSOD and DVPSe caused an increase of the latter.

Modulation of doxorubicin-induced genotoxicity by Aegle marmelos in mouse bone marrow: a micronucleus study

The effect of various concentrations of Aegle marmelos (AME) on the doxorubicin (DOX)-induced genotoxic effects in mice bone marrow was studied. Treatment of mice with different concentrations of DOX resulted in a dose-dependent elevation in the frequency of micronucleated polychromatic (MPCE) as well as normochromatic (MNCE) erythrocytes in mouse bone marrow. The frequencies of MPCE and MNCE increased with scoring time, and the greatest elevation for MPCE was observed at 48 hours post-DOX treatment, whereas a maximum increase in MNCE was observed at 72 hours post-DOX treatment. This increase in MPCE and MNCE was accompanied by a decline in the polychromatic erythrocytes-normochromatic erythrocytes (PCE/NCE) ratio, which showed a DOX-dose-dependent decline. Treatment of mice with 200, 250, 300, 350, and 400 mg/kg body weight of AME, orally once daily for 5 consecutive days before DOX treatment, significantly reduced the frequency of DOX-induced micronuclei accompanied by a significant elevation in the PCE/NCE ratio at all scoring times. The greatest protection against DOX-induced genotoxicity was observed at 350 mg/kg AME. The protection against DOX-induced genotoxicity by AME may be due to inhibition of free radicals and increased antioxidant status.

Thiocolchicine dimers: a novel class of topoisomerase-I inhibitors

During a cellular screening of thiocolchicine analogs, thiocolchicine dimers resulted particularly active in cisplatin-resistant A2780-CIS cells. In order to discover by which mechanism(s) thiocolchicine dimers overcame cisplatin resistance, p53, p21waf1 and MLH1 were assessed by Western blot. Results pointed out that, when combined with cisplatin, dimers increased the amount of all the three proteins with respect to the levels obtained by single drug exposure, thereby suggesting an interference in the process of repair of the cisplatin-induced DNA lesions. Moreover, in isolated nuclei drugs were able to produce DNA breaks, as demonstrated by Comet assay, thereby proving that the compounds were able to target cell nucleus independently from microtubules. Since Topo-I (topoisomerase I) is directly involved in the DNA repair and such activity is overexpressed in cisplatin-resistant cells, Topo-I was investigated as a potential target. Using DNA relaxation assay, thiocolchicine dimers inhibited Topo-I, a property not shared by thiocolchicine. At variance with camptothecin, dimers did not produce cleavable complexes, thereby indicating that Topo-I inhibition occurs upstream of the religation step. To assess the mechanism of inhibition, an electrophoretic mobility shift assay between DNA and Topo-I was performed and revealed that thiocolchicine dimers specifically interfere with binding of Topo-I to DNA. The interference is specific since the same compounds did not modulate DNase activity and did not act as intercalating agents in the DNA unwinding assay. Finally, behaviour of dimers as spindle poisons was investigated and no relevant changes with respect to thiocolchicine in terms of interaction with microtubules were found.

Tumor acidity, ion trapping and chemotherapeutics. I. Acid pH affects the distribution of chemotherapeutic agents in vitro

Resistance to anti-cancer chemotherapies often leads to regional failure, and can be caused by biochemical and/or physiological mechanisms. Biochemical mechanisms include the overexpression of resistance-conferring proteins. In contrast, physiological resistance involves the tumor microenvironment, and can be caused by poor perfusion, hypoxia and/or acidity. This communication investigates the role of tumor acidity in resistance to a panel of chemotherapeutic agents commonly used against breast cancer, such as anthracyclines, taxanes, anti-metabolites and alkylating agents. The effects of pH on the cytotoxicity of these agents were determined, and ion trapping was confirmed by monitoring the effect of pH on the cellular uptake of radiolabeled anthracyclines. Furthermore, pH-dependent cytotoxicity and uptake were compared between parental drug sensitive MCF-7 cells and variants overexpressing p-glycoprotein (MDR-1) and Breast Cancer Resistance Protein. These data indicate that the magnitude of physiological resistance from pH-dependent ion trapping is comparable to biochemical resistance caused by overexpression of drug efflux pumps. Hence, microenvironment-based ion trapping is a significant barrier to anthracycline-based chemotherapy and can itself be a therapeutic target to enhance the efficacy of existing chemotherapies.

Tumor acidity, ion trapping and chemotherapeutics. II. pH-dependent partition coefficients predict importance of ion trapping on pharmacokinetics of weakly basic chemotherapeutic agents

Ion-trapping theory predicts that alkalinization of tumor extracellular pH will enhance the anti-tumor activity of weak-base chemotherapeutics. We have previously demonstrated that chronic and acute treatment of tumor-bearing mice with sodium bicarbonate results in tumor-specific alkalinization of extracellular pH. Furthermore, bicarbonate pretreatment enhances the anti-tumor activity of doxorubicin and mitoxantrone in two different mouse tumor models. Previous work has indicated subtle, yet significant differences between the pH sensitivities of the biodistribution and anti-tumor efficacies of doxorubicin and mitoxantrone in vitro. The present study demonstrates that systemic alkalinization selectively enhances tumor uptake of radiolabeled mitoxantrone, but not doxorubicin. Results using these two drugs are quantitatively and qualitatively very different, and can be explained on the basis of differences in the octanol-water partition coefficients of their charged forms. These results suggest that inducing metabolic alkalosis in patients would have a positive effect on response to mitoxantrone therapy. However, the therapeutic index would not increase if sodium bicarbonate also caused increased retention of mitoxantrone in susceptible normal tissues in the host. The major dose-limiting organ systems for mitoxantrone are heart, liver, bone marrow, spleen and blood cells. Bicarbonate was found to have no significant effect on the distribution of mitoxantrone to any of these tissues except for spleen. However, neither spleen weights nor lymphocyte counts were adversely affected by NaHCO(3) pretreatment, indicating that this co-therapy does not enhance myelosuppression due to mitoxantrone therapy. These findings suggest that metabolic alkalosis would produce a net gain in mitoxantrone therapeutic index.

Position-specific trapping of topoisomerase II by benzo[a]pyrene diol epoxide adducts: implications for interactions with intercalating anticancer agents

DNA topoisomerase II (Top2) is the target of some of the most effective anticancer DNA intercalators. To determine the effect of intercalating ligands at defined positions relative to a known DNA cleavage site for human Top2alpha, we synthesized oligodeoxynucleotides containing single trans-opened benzo[a]pyrene 7,8-diol 9,10-epoxide (DE) deoxyadenosine (dA) adducts of known absolute configuration, placed at specific positions in a duplex sequence containing staggered Top2 cleavage sites on both strands. Because the orientations of the intercalated hydrocarbon are known from NMR solution structures of duplex oligonucleotides containing these dA adducts, a detailed analysis of the relationship between the position of intercalation and trapping of Top2 is possible. Our findings demonstrate that (i) Top2 cleavage complexes are trapped by intercalation of the hydrocarbon at either of the staggered cleavage sites or immediately adjacent to the base pairs flanking the cleavage sites within the stagger; (ii) both concerted and nonconcerted cleavage by both subunits of a Top2 homodimer were detected depending on the position of the benzo[a]pyrene DE dA adduct; and (iii) intercalation immediately outside of the staggered Top2 cleavage site, and to a lesser extent in the middle of the stagger, prevents Top2 from cleaving DNA at this site, consistent with the effect of some intercalators as suppressors of Top2-mediated DNA cleavage. These results identify specific binding sites for intercalators that result in trapping of Top2. Such poisoning of Top2 by bulky polycyclic aromatic hydrocarbon DE adducts constitutes a potential mechanism for their carcinogenic activity.

hTERT associates with human telomeres and enhances genomic stability and DNA repair

Ectopic expression of telomerase in telomerase-silent cells is sufficient to overcome senescence and to extend cellular lifespan. We show here that the catalytic subunit of human telomerase (hTERT) crosslinks telomeres. This interaction is blocked by the telomere repeat binding factor 1, but not by a dominant negative form of this protein. It is also abolished by destruction of the RNA component of telomerase as well as by mutations in the hTERT protein. Ectopic expression of hTERT leads to transcriptional alterations of a subset of genes and changes in the interaction of the telomeres with the nuclear matrix. This is associated with reduction of spontaneous chromosome damage in G(1) cells, enhancement of the kinetics of DNA repair and an increase in NTP levels. The effect on DNA repair is likely indirect as TERT does not directly affect DNA end rejoining in vitro or meiotic recombination in vivo. The observed effects of hTERT occurred rapidly before any significant lengthening of telomeres was observed. Our findings establish an intimate relationship between hTERT-telomere interactions and alteration in transcription of a subset of genes that may lead to increased genomic stability and enhanced repair of genetic damage. These novel functions of telomerase are distinct from its known effect on telomere length and have potentially important biological consequences.

A novel carbazole topoisomerase II poison, ER-37328: potent tumoricidal activity against human solid tumors in vitro and in vivo

We have discovered a novel topoisomerase II (topo II) poison, ER-37328 (12,13-dihydro-5-[2-(dimethylamino)ethyl]-4H-benzo[c]pyrimido[5,6,1-jk]carbazole-4,6,10(5H,11H)-trione hydrochloride), which shows potent tumor regression activity against Colon 38 cancer inoculated s.c. Here, we describe studies on the cell-killing activity against a panel of human cancer cell lines and the antitumor activity of ER-37328 against human tumor xenografts. In a cell-killing assay involving 1-h drug treatment, ER-37328 showed more potent cell-killing activity (50% lethal concentrations (LC50s) ranging from 2.9 to 20 microM) than etoposide (LC50s>60 microM) against a panel of human cancer cell lines. ER-37328 induced double-stranded DNA cleavage, an indicator of topo II-DNA cleavable complex formation, within 1 h in MX-1 cells, and the extent of cleavage showed a bell-shaped relationship to drug concentration, with the maximum at 2.5 microM. After removal of the drug (2.5 microM) at 1 h, incubation was continued in drug-free medium, and the amount of cleaved DNA decreased. However, at 10 microM, which is close to the LC50s against MX-1 cells, DNA cleavage was not detected immediately after 1-h treatment, but appeared and increased after drug removal. This result may explain the potent cell-killing activity of ER-37328 in the 1-h treatment. In vivo, ER-37328 showed potent tumor regression activity against MX-1 and NS-3 tumors. Moreover, ER-37328 had a different antitumor spectrum from irinotecan or cisplatin against human tumor xenografts. In conclusion, ER-37328 is a promising topo II poison with strong cell killing activity in vitro and tumor regression activity in vivo, and is a candidate for the clinical treatment of malignant solid tumors.

Structurally different anthracyclines provoke different effects on cell cycle and tumor B cell differentiation

Previously we have detected a stimulatory effect on immunoglobulin (IgG) synthesis when hybridoma cells were treated with doxorubicin. In order to determine whether this is a general property of anthracycline, we have selected three analogs--doxorubicin (DOX), pirarubicin (THP-DOX) and aclarubicin (ACR)--which differ mainly in the methylation state of their amino sugars. Cell cycle analysis by flow cytometry and drug localization by scanning confocal microscopy were also performed. The results show that when cells (UN2 hybridoma B cells), were exposed to subtoxic doses of DOX or THP (with unmethylated amino sugars), a strong increases in IgG secretion, heavy (H) and light (L) chain synthesis and the corresponding mRNA levels were induced. Furthermore these two drugs arrested the cells in the G2/M phase of the cell cycle. In contrast, exposure to ACR (with its methylated amino sugar) at similar subtoxic doses induced a blockade of cells in the G1 phase with no increase of IgG synthesis, at the subtoxic doses used, all three drugs could still be detected in the nucleus as well as in the cytoplasm, as determined by confocal laser microscopy. Thus, the relationship between cell cycle blockade, IgG stimulation and anthracycline structure is suggested by these results.

Synthesis and biological properties of a new series of N-pyrido substituted tetrahydrocarbazoles

A series of methyl and ethyl quaternary pyridiniumtetrahydrocarbazoles was synthesized and studied in comparison with ellipticine, chosen as a reference. In general, their antiproliferative activity, tested in different biological substrates, appeared to be higher than that of the corresponding non-quaternarized compounds. This fact could be attributed to the introduction of a positive charge in the molecule, which can stabilize the molecular complex they form with DNA. In a prokaryotic system, the T2 bacteriophage, both quaternarized and non-quaternarized compounds inhibited its infectivity moderately, in a similar way to ellipticine. This effect seemed to be connected to a direct activity on the virions rather than on the indicator bacteria. In mammalian cells, the pyridiniumtetrahydrocarbazoles were more effective. In particular, they appeared to be very active in inhibiting DNA synthesis in Ehrlich ascites cells; some of them were as effective as ellipticine. However, pyridiniumtetrahydrocarbazoles were less active in comparison with ellipticine when their capacity for inhibiting the clonal growth in Chinese hamster ovary (CHO) cells was tested. A similar picture was obtained studying the formation of chromosome aberrations and of sister chromatid exchanges in the same cells. These different responses can be explained considering that the data on DNA synthesis reflect effects only on DNA replication within a short time, without considering any later consequences; on the contrary, in the long-term tests, other events, which lead to cell killing or genotoxicity, can take place. Pyridiniumtetrahydrocarbazoles damage DNA, inducing double-strand breaks efficiently. These observations, together with the data already obtained on unsubstituted derivatives, suggest the pyridiniumtetrahydrocarbazoles induce antiproliferative and genotoxic effects, very probably by inhibiting topoisomerase II.

Relationship between lethal effects and topoisomerase II-mediated double-stranded DNA breaks produced by anthracyclines with different sequence specificity

The role of the site selectivity of topoisomerase II poisoning in the cytotoxic activity of anthracyclines has not been established. In this article, we have thus studied the levels and persistence of double-stranded DNA breaks (DSB) along with the cytotoxic activity in human leukemic HL60 cells of seven anthracyclines, including doxorubicin, daunorubicin, and idarubicin, as well as sugar-modified analogues characterized by an altered sequence specificity. Epimerization at the 3' position of the sugar moiety markedly affected the biological activity; indeed, a dramatic reduction of drug effects was evident for 3'-deamino-3'-epi-hydroxy-4'-deoxy-4'-amino-daunorubicin. The studied analogues could be gathered into three groups based on the DSB/cytotoxicity ratio. At equitoxic concentrations: (a) parent drugs and 3'-deamino-3'-epi-hydroxy-4'-deoxy-4'-amino-daunorubicin endowed with the same sequence specificity stimulated low DSB levels; (b) 3'-epi-daunorubicin and 3'-deamino-4'-deoxy-4'-epi-amino-idarubicin, which have a different sequence specificity, and teniposide (a structurally unrelated poison) stimulated higher amounts of DSB; and (c) 4-demethoxy-3'-deamino-3'-hydroxy-4'-epi-doxorubicin stimulated the highest DSB levels. For the last agent, a faster rate of cleavage resealing, which is consistent with a reduced DNA binding affinity, could account for the increased DSB/cytotoxicity ratio compared with parent drugs. However, for other analogues, the observed differences in DSB persistence/resealing could not completely explain the different DSB/cytotoxicity ratios. The results thus suggest that the cytotoxic potency of anthracyclines may be the result of an interplay of the level, the persistence, and the genomic localization of topoisomerase II-mediated DNA cleavage.

Correlation between the formation of cleavable complex with topoisomerase I and growth-inhibitory activity for saintopin-type antibiotics

New saintopin-type antibiotics (e.g., saintopin, saintopin E, UCE1022, UCE6) with a naphthacene-dione structure have been discovered through our mechanistically oriented screening using purified mammalian DNA topoisomerases. Saintopin is a dual inducer of topoisomerase I- and topoisomerase II-mediated DNA cleavages in a cell-free system using purified enzymes, whereas others induced topoisomerase I- but not topoisomerase II-mediated DNA cleavage. The order of topoisomerase I-mediated DNA cleavage activity at lower concentrations (<1 microM) was UCE6 > saintopin > saintopin E > UCE1022. The DNA cleavage-intensity patterns induced by these antibiotics with topoisomerase I were identical, indicating that saintopin-type antibiotics have a similar DNA sequence selectivity in stabilization of the cleavable complex with topoisomerase I. Increases in protein/DNA complexes were observed in saintopin-type antibiotic-treated HeLa S3 cells using the potassium/sodium dodecyl sulfate precipitation method. Brief heating of these drugs-treated cells at 65 degrees for 10 min resulted in a rapid reduction in the number of protein/DNA complexes. Immunoblot analysis using antibody against human topoisomerase I or II revealed that the protein linked to DNA in saintopin-type antibiotic-treated cells is most likely topoisomerase I. These results suggest that saintopin-type antibiotics interfere with topoisomerase I in cells by trapping reversible topoisomerase I/DNA cleavable complexes. The formation of topoisomerase I/DNA complexes by saintopin-type antibiotics correlates well with their growth-inhibitory activities, suggesting that topoisomerase I can be the principal target of these antibiotics.

Comparison of aza-anthracenedione-induced DNA damage and cytotoxicity in experimental tumor cells

Aza-anthracenediones are a new class of anti-cancer drugs, which demonstrate promising in vitro and in vivo activity. Our laboratory has synthesized a variety of structural analogs in which we determined previously that the positioning of the nitrogen within the backbone, as well as sidearm modification, results in dramatic differences in the potency of cytotoxicity. We reported previously that although DNA reactivity appears to be a necessary component for mediating cell death, it is not sufficient for predicting cytotoxicity of the aza-anthracenediones. We have chosen three aza-anthracenediones (BBR 2828, BBR 2778 and BBR 2378) to investigate the importance of DNA strand breaks and/or protein-concealed DNA breaks induced by aza-anthracenediones. We determined in the present study that, while all three drugs cause DNA breaks as determined by alkaline and neutral elution, as well as KCl-SDS precipitation, these breaks do not correlate directly with their potency as cytotoxic compounds. Further, we found significant differences in the types of DNA breaks induced by these drugs. Finally, we report that the persistence of protein-DNA complexes induced by all three drugs was similar and, therefore, cannot account for differences in the potency of cytotoxicity of the aza-anthracenediones. Thus, we postulate that, while the total number of drug-induced protein-concealed DNA breaks is an important indicator of drug toxicity, it is possible that the actual nature of the breaks may differ among the aza-anthracenedione congeners, and it is these differences in the actual proteins present in the DNA breaks that differentiate between aza-anthracenediones.

Topoisomerase II binds to ellipticine in the absence or presence of DNA. Characterization of enzyme-drug interactions by fluorescence spectroscopy

Although a number of drugs currently in use for the treatment of human cancers act by stimulating topoisomerase II-mediated DNA breakage, little is known regarding interactions between these agents and the enzyme. To further define the mechanism of drug action, interactions between ellipticine (an intercalative drug with clinical relevance) and yeast topoisomerase II were characterized. By utilizing a yeast genetic system, topoisomerase II was identified as the primary cellular target of the drug. Furthermore, ellipticine did not inhibit enzyme-mediated DNA religation, suggesting that it stimulates DNA breakage by enhancing the forward rate of cleavage. Finally, ellipticine binding to DNA, topoisomerase II, and the enzyme-DNA complex was assessed by steady-state and frequency domain fluorescence spectroscopy. As determined by changes in fluorescence intensity and emission maximum wavelength, and by lifetime analysis, only the protonated species of ellipticine bound to a double-stranded 40-mer oligonucleotide containing a topoisomerase II cleavage site (KD approximately 65 nM). In contrast, predominantly deprotonated ellipticine bound to the enzyme.DNA complex (KD approximately 1.5 microM) or to the enzyme in the absence of nucleic acids (KD approximately 160 nM). These findings suggest that ellipticine interacts directly with topoisomerase II and that the enzyme dictates the ionic state of the drug in the ternary complex. A model is presented in which the topoisomerase II.ellipticine.DNA complex is formed via initial drug binding to either the enzyme or DNA.

Correlation of DNA reactivity and cytotoxicity of a new class of anticancer agents: aza-anthracenediones

Doxorubicin and mitoxantrone are carboxyclic anti-cancer drugs that interact with DNA through intercalation. Our laboratory has synthesized a new series of anti-tumor agents, the aza-anthracenediones, which are structurally related to mitoxantrone but contain a heterocyclic, rather than a carbocyclic, chromophore. Both the in vivo and in vitro anti-tumor activities of these compounds were exquisitely sensitive to the positioning of the nitrogen atom within the heterocyclic backbone. Compounds having a 2-aza were 30- to 100-fold more potent than the 1-aza or the di-aza compounds against L1210 cells in vitro. When tested in vivo, the 2-aza-anthracenediones had marked anti-tumor activity, in some cases curative, whereas the 1-aza-anthracenediones had but minimal antitumor activity. To define the importance of the aza positioning on DNA reactivity, spectral shift and gel mobility assays were used. The spectral shift assay suggested that the 2-aza compounds reacted with DNA solely through intercalation whereas the 1-aza-anthracenediones, and mitoxantrone all reacted with DNA through intercalative and non-intercalative processes. The affinity of DNA binding was five to seven times greater for the 2-aza compounds compared to the 1-aza or the di-aza derivatives. The retardation of supercoiled pBR322 DNA mobility in agarose gel electrophoresis further suggested an intercalative type of DNA interaction. Differences in DNA interaction appear related to but can not completely account for differences in cytotoxicity of the aza anthracenediones.

Base sequence determinants of amonafide stimulation of topoisomerase II DNA cleavage

A number of antitumor drugs including naphthalimides, a new class of intercalating agents, interfere with the DNA breakage-reunion activity of mammalian DNA topoisomerase II resulting in DNA cleavage stimulation. In this work, the sequence specificity of a lead compound of this series, amonafide, in stimulating DNA cleavage by murine topoisomerase II has been studied. Amonafide-stimulated cleavage intensity patterns were markedly different from those of other antitumor drugs by using pBR322 and SV40 DNAs. This drug had an unusually high site selectivity since about 60% of DNA cleavage was observed at only one site in pBR322 DNA, and at two sites in SV40 DNA. A total of ninety-four drug-stimulated sites were collected, and a statistical analysis of their sequences showed that amonafide highly prefers a cytosine, and excludes guanines and thymines instead, at position -1. A lower preference for an adenine at position +1 was also noted. In agreement with the statistical analysis, the DNA sequences of the three sites stimulated by amonafide at exceptionally high levels showed that the drug requirements of a cytosine (-1) and adenine (+1) were present in both the two strands. In addition, a particular feature of these prominent cleavage sites was the presence of an inverted repeat from position -3 to +7. Comparison of amonafide stimulation of DNA cleavage in oligonucleotides bearing base mutations at positions -2, -3 and/or +6, +7 suggested that DNA sequence, and not a putative cruciform structure, was critical for drug action. Moreover, the results showed that, for strong cleavage stimulation, the primary drug requirements at -1 and +1 positions were not sufficient and that the sequence 5'-WRC decreases A-3' (W, A or T; R, A or G) is required from -3 to +1 positions at both strands. The results suggest that the exceptionally high sequence specificity of amonafide is the result of optimal drug interactions with both the two enzyme subunits.

In vitro effects of pentoxifylline and doxorubicin on cell survival and DNA damage in sensitive and MDR-P388 leukemia cells

The utility of chemosensitizers to improve efficacy of chemotherapy is now gaining importance. This report investigated whether an active hemorheological agent, pentoxifylline (PTX), can circumvent drug resistance in parental (P388/S) and multidrug resistant (P388/DOX) P388 leukemia cells. For detection of doxorubicin (DOX) resistance and reversal of this resistance by PTX, the incorporation of nucleic acid precursor was measured after addition of DOX and PTX, respectively. The effect of PTX on the induction of DNA strand breaks by DOX was also examined. Increased fragmentation of DNA was illustrated in P388/DOX leukemia cells exposed to the combination of DOX and PTX. The most prominent feature of the multidrug-resistant cell is the reduced accumulation of the drug intracellularly. P388/DOX cells showed less accumulation of DOX in the cell as compared to that of the parental cell line. Further studies demonstrated that PTX significantly enhanced the intracellular accumulation of DOX in both the cell lines. These studies warrant the use of PTX as an adjuvant in cancer chemotherapy.

Defining functional drug-interaction domains on topoisomerase II by exploiting mechanistic differences between drug classes

Topoisomerase II is the primary cellular target for a variety of antineoplastic drugs that are active against human cancers. These drugs exert their cytotoxic effects by stabilizing covalent topoisomerase II-cleaved DNA complexes that are fleeting intermediates in the catalytic cycle of the enzyme. Despite this common feature of drug action, a number of mechanistic differences between drug classes have been described. These mechanistic differences (including effects on DNA cleavage/religation, DNA strand passage, and adenosine triphosphate hydrolysis) were used as the basis for a series of competition experiments to determine whether different compounds share a common site of action on topoisomerase II or interact at distinct sites. Results of the present study strongly suggest that at least four structurally disparate antineoplastic drugs, etoposide, amsacrine, genistein, and the quinolone CP-115,953, share an overlapping interaction domain on the enzyme.

Increased sensitivity of damaged DNA to digestion with nuclease S1 as assessed in single cells by flow cytometry

DNA sensitivity to digestion with nuclease S1 was investigated in cells irradiated with gamma rays, or treated with the antitumor drug adriamycin (Adr). The nuclease-resistant DNA fraction was determined by propidium iodide staining. Treated cells were found to be more sensitive to nuclease digestion than the undamaged controls. Gamma ray-induced strand breaks were detectable at doses up to 10 Gy; an increase in the reaction temperature, from 37 degrees to 63 degrees C, was necessary in order to detect higher levels of damage. Nuclease S1 sensitivity in Adr-treated cells showed a single-peak, concentration-dependent relationship, in agreement with the known self-inhibitory effect exerted by high drug doses. Determination of DNA digestion could be performed in combination with other cellular parameters (e.g., protein content). Detection of drug-resistant cells in a heterogeneous population of small-cell lung carcinoma was achieved on the basis of the different sensitivity of the cells to enzymatic digestion. These results indicate that nuclease S1 may be a useful probe for studying in single cells DNA alterations induced by drugs or radiation.

Isolation of cDNA clones encoding the beta isozyme of human DNA topoisomerase II and localisation of the gene to chromosome 3p24

Topoisomerases catalyse the interconversion of topological isomers of DNA and have key roles in nucleic acid metabolism. Human cells express two distinct type II topoisomerase isozymes, designated topoisomerase II alpha (170 kDa form) and topoisomerase II beta (180 kDa form). We have isolated cDNA clones encoding the beta isozyme from a human B-cell library. The proposed coding region for the topoisomerase II beta protein is 4,863 nucleotides long and would encode a polypeptide with a calculated M(r) of 182,705. The predicted topoisomerase II beta protein sequence shows striking similarity (72% identical residues) to that of the human alpha isozyme, and homology to topoisomerase II proteins from Drosophila, yeast and bacteria. Regions of greatest amino acid sequence divergence lie at the extreme N-terminus and over a C-terminal domain comprising approximately 25% of the total protein. We have quantified the level of topoisomerase II beta mRNA in a panel of human tumour cell lines of different origin using an RNase protection assay, and compared the level to that of topoisomerase II alpha mRNA. Topoisomerase II beta mRNA was expressed in haemopoietic, epithelial and fibroblast cell lines, although to different extents, with U937 cells (promonocytic leukaemia) showing a particularly high level. There was no obvious relationship in terms of level of expression between the topoisomerase II alpha and beta genes. We have localised the gene encoding topoisomerase II beta protein to chromosome 3p24 in the human genome.

Induction of chromosomal aberrations by camptothecin in root-tip cells of Vicia faba

When root-tip cells of Vicia faba were exposed during early and middle interphase to camptothecin (Cpt), the aberrations obtained were exclusively of the chromatid type and tended to be localized in late replicating heterochromatic regions of the chromosomes. In these respects the clastogenic effect of Cpt resembles that of agents that act by an S-phase-dependent mechanism. In contrast to typical S-phase-dependent agents, Cpt produced lesions capable of giving rise to aberrations only in S-phase cells that were synthesizing DNA at the time of treatment. The dependence on ongoing DNA synthesis was suggested in autoradiographic experiments and by the fact that the clastogenic effect of Cpt was strongly suppressed by hydroxyurea, an inhibitor of DNA synthesis. After Cpt treatments, there were many more cells with 3-12 aberrations and far fewer cells with 0, 1 or 2 aberrations than expected on the basis of a Poisson distribution. Cpt further differed from typical S-phase-dependent agents by being capable of inducing lesions in the G2 phase that give rise to chromosomal aberrations in the first mitosis after treatment. This effect was obtained at Cpt concentrations around 10 microM, whereas only 0.03 microM Cpt was required to produce chromatid aberrations in the S phase. Results of G2-phase experiments with Cpt and 5-fluorodeoxyuridine, an inhibitor of DNA synthesis, suggest that DNA synthesis is required for the clastogenic effect of Cpt not only during the S phase, but also during the G2 phase, although the DNA syntheses involved are both quantitatively and qualitatively different.