Free radical mechanisms in neocarzinostatin-induced DNA damage

Identifying Suitable Reference Gene Candidates for Quantification of DNA Damage-Induced Cellular Responses in Human U2OS Cell Culture System

DNA repair pathways trigger robust downstream responses, making it challenging to select suitable reference genes for comparative studies. In this study, our goal was to identify the most suitable housekeeping genes to perform comparable molecular analyses for DNA damage-related studies. Choosing the most applicable reference genes is important in any kind of target gene expression-related quantitative study, since using the housekeeping genes improperly may result in false data interpretation and inaccurate conclusions. We evaluated the expressional changes of eight well-known housekeeping genes (i.e., 18S rRNA, B2M, eEF1α1, GAPDH, GUSB, HPRT1, PPIA, and TBP) following treatment with the DNA-damaging agents that are most frequently used: ultraviolet B (UVB) non-ionizing irradiation, neocarzinostatin (NCS), and actinomycin D (ActD). To reveal the significant changes in the expression of each gene and to determine which appear to be the most acceptable ones for normalization of real-time quantitative polymerase chain reaction (RT-qPCR) data, comparative and statistical algorithms (such as absolute quantification, Wilcoxon Rank Sum Test, and independent samples T-test) were conducted. Our findings clearly demonstrate that the genes commonly employed as reference candidates exhibit substantial expression variability, and therefore, careful consideration must be taken when designing the experimental setup for an accurate and reproducible normalization of RT-qPCR data. We used the U2OS cell line since it is generally accepted and used in the field of DNA repair to study DNA damage-induced cellular responses. Based on our current data in U2OS cells, we suggest using 18S rRNA, eEF1α1, GAPDH, GUSB, and HPRT1 genes for UVB-induced DNA damage-related studies. B2M, HPRT1, and TBP genes are recommended for NCS treatment, while 18S rRNA, B2M, and PPIA genes can be used as suitable internal controls in RT-qPCR experiments for ActD treatment. In summary, this is the first systematic study using a U2OS cell culture system that offers convincing evidence for housekeeping gene selection following treatment with various DNA-damaging agents. Here, we unravel an indispensable issue for performing and assessing trustworthy DNA damage-related differential gene expressional analyses, and we create a "zero set" of potential reference gene candidates.

Cleavage of Early Mouse Embryo with Damaged DNA

The preimplantation period of embryogenesis is crucial during mammalian ontogenesis. During this period, the mitotic cycles are initiated, the embryonic genome is activated, and the primary differentiation of embryonic cells occurs. All cellular abnormalities occurring in this period are the primary cause of fetal developmental disorders. DNA damage is a serious cause of developmental failure. In the context of DNA damage response on the cellular level, we analyzed the course of embryogenesis and phenotypic changes during the cleavage of a preimplantation embryo. Our results document that DNA damage induced before the resumption of DNA synthesis in a zygote can significantly affect the preimplantation development of the embryo. This developmental ability is related to the level of the DNA damage. We showed that one-cell embryos can correct the first cleavage cycle despite low DNA damage and incomplete replication. It seems that the phenomenon creates a predisposition to a segregation disorder of condensed chromatin that results in the formation of micronuclei in the developmental stages following the first cleavage. We conclude that zygote tolerates a certain degree of DNA damage and considers its priority to complete the first cleavage stage and continue embryogenesis as far as possible.

Deciphering Nanoparticle Trafficking into Glioblastomas Uncovers an Augmented Antitumor Effect of Metronomic Chemotherapy

Nanoparticles have been explored in glioblastomas as they can traverse the blood-brain barrier and target glioblastoma selectively. However, direct observation of nanoparticle trafficking into glioblastoma cells and their underlying intracellular fate after systemic administration remains uncharacterized. Here, based on high-resolution transmission electron microscopy experiments of an intracranial glioblastoma model, it is shown that ligand-modified nanoparticles can traverse the blood-brain barrier, endocytose into the lysosomes of glioblastoma cells, and undergo endolysosomal escape upon photochemical ionization. Moreover, an optimal dose of metronomic chemotherapy using dual-drug-loaded nanocarriers can induce an augmented antitumor effect directly on tumors, which has not been recognized in previous studies. Metronomic chemotherapy enhances antitumor effects 3.5-fold compared with the standard chemotherapy regimen using the same accumulative dose in vivo. This study provides a conceptual framework that can be used to develop metronomic nanoparticle regimens as a safe and viable therapeutic strategy for treating glioblastomas and other advanced-stage solid tumors.

Oxidative Stress: Role and Response of Short Guanine Tracts at Genomic Locations

Over the decades, oxidative stress has emerged as a major concern to biological researchers. It is involved in the pathogenesis of various lifestyle-related diseases such as hypertension, diabetes, atherosclerosis, and neurodegenerative diseases. The connection between oxidative stress and telomere shortening via oxidative guanine lesion is well documented. Telomeres are confined to guanine rich ends of chromosomes. Owing to its self-association properties, it adopts G-quadruplex structures and hampers the overexpression of telomerase in the cancer cells. Guanine, being the most oxidation prone nucleobase, when structured in G-quadruplex entity, is found to respond peculiarly towards oxidative stress. Interestingly, this non-Watson-Crick structural feature exists abundantly in promoters of various oncogenes, exons and other genomic locations. The involvement of G-quadruplex architecture in oncogene promoters is well recognized in gene regulation processes. Development of small molecules aimed to target G-quadruplex structures, have found to alter the overexpression of oncogenes. The interaction may lead to the obstruction of diseased cell having elevated level of reactive oxygen species (ROS). Thus, presence of short guanine tracts (Gn) forming G-quadruplexes suggests its critical role in oxidative genome damage. Present review is a modest attempt to gain insight on the association of oxidative stress and G-quadruplexes, in various biological processes.

A pro longevity role for cellular senescence

Cellular senescence is a fundamental process that may play positive or detrimental roles for the organism. It is involved in tissue development and in tumor prevention although during aging is becoming a detrimental process contributing to the decline of tissue functions. In previous investigations, we have uncovered a better capacity to detect DNA damage in cells from long-lived mammals. Here, we report that cultured cells derived from long-lived species have a higher propensity to undergo senescence when challenged with DNA damage than cells derived from short-lived species. Using a panel of cells derived from six mammals, which range in lifespan from 3-4 years up to 120 years, we examined cell cycle response, induction of apoptosis and of cellular senescence. All species exhibited a cell cycle arrest while induction of apoptosis was variable. However, a significant positive correlation was found between the relative percent of cells, within a population which entered senescence following damage, and the lifespan of the species. We suggest that cellular senescence may have a positive role during development allowing it to contribute to the evolution of longevity.

DNA Damage Detection by 53BP1: Relationship to Species Longevity

In order to examine potential differences in genomic stability, we have challenged fibroblasts derived from five different mammalian species of variable longevity with the genotoxic agents, etoposide and neocarzinostatin. We report that cells from longer-lived species exhibit more tumor protein p53 binding protein 1 (53BP1) foci for a given degree of DNA damage relative to shorter-lived species. The presence of a greater number of 53BP1 foci was associated with decreased DNA fragmentation and a lower percentage of cells exhibiting micronuclei. These data suggest that cells from longer-lived species have an enhanced DNA damage response. We propose that the number of 53BP1 foci that form in response to damage reflects the intrinsic capacity of cells to detect and respond to DNA harms.

End-processing nucleases and phosphodiesterases: An elite supporting cast for the non-homologous end joining pathway of DNA double-strand break repair

Nonhomologous end joining (NHEJ) is an error-prone DNA double-strand break repair pathway that is active throughout the cell cycle. A substantial fraction of NHEJ repair events show deletions and, less often, insertions in the repair joints, suggesting an end-processing step comprising the removal of mismatched or damaged nucleotides by nucleases and other phosphodiesterases, as well as subsequent strand extension by polymerases. A wide range of nucleases, including Artemis, Metnase, APLF, Mre11, CtIP, APE1, APE2 and WRN, are biochemically competent to carry out such double-strand break end processing, and have been implicated in NHEJ by at least circumstantial evidence. Several additional DNA end-specific phosphodiesterases, including TDP1, TDP2 and aprataxin are available to resolve various non-nucleotide moieties at DSB ends. This review summarizes the biochemical specificities of these enzymes and the evidence for their participation in the NHEJ pathway.

Effects of melatonin on the nitric oxide system and protein nitration in the hypobaric hypoxic rat hippocampus

BACKGROUND

It is well documented that the nitric oxide (NO) might be directly involved in brain response to hypobaric hypoxia, and could contribute to memory deficiencies. Recent studies have shown that melatonin could attenuate hypoxia or ischemia-induced nerve injuries by decreasing the production of free radicals. The present study, using immunohistochemical and immunoblot methods, aimed to explore whether melatonin treatment may affect the expression of nitric oxide system and protein nitration, and provide neuroprotection in the rat hippocampus injured by hypobaric hypoxia. Prior to hypoxic treatment, adult rats were pretreated with melatonin (100 mg/kg, i.p.) before they were exposed to the altitude chamber with 48 Torr of the partial oxygen concentration (pO2) for 7 h to mimic the ambience of being at 9000 m in height. They were then sacrificed after 0 h, 1, and 3 days of reoxygenation.

RESULTS

The results obtained from the immunohistochemical and immunoblotting analyses showed that the expressions of neuronal nitric oxide synthase (nNOS), endothelial nitric oxide synthase (eNOS), inducible nitric oxide synthase (iNOS), nitrotyrosine (Ntyr) and Caspase 3 in the hypoxic hippocampus were increased from 0 h to 3 days of reoxygenation. Interestingly, the hypoxia-induced increase of nNOS, eNOS, iNOS, Ntyr and Caspase 3 protein expression was significantly depressed in the hypoxic rats treated with melatonin.

CONCLUSIONS

Activation of the nitric oxide system and protein nitration constitutes a hippocampal response to hypobaric hypoxia and administration of melatonin could provide new therapeutic avenues to prevent and/or treat the symptoms produced by hypobaric hypoxia.

Canonical non-homologous end joining in mitosis induces genome instability and is suppressed by M-phase-specific phosphorylation of XRCC4

DNA double-strand breaks (DSBs) can be repaired by one of two major pathways-non-homologous end-joining (NHEJ) and homologous recombination (HR)-depending on whether cells are in G1 or S/G2 phase, respectively. However, the mechanisms of DSB repair during M phase remain largely unclear. In this study, we demonstrate that transient treatment of M-phase cells with the chemotherapeutic topoisomerase inhibitor etoposide induced DSBs that were often associated with anaphase bridge formation and genome instability such as dicentric chromosomes. Although most of the DSBs were carried over into the next G1 phase, some were repaired during M phase. Both NHEJ and HR, in particular NHEJ, promoted anaphase-bridge formation, suggesting that these repair pathways can induce genome instability during M phase. On the other hand, C-terminal-binding protein interacting protein (CtIP) suppressed anaphase bridge formation, implying that CtIP function prevents genome instability during mitosis. We also observed M-phase-specific phosphorylation of XRCC4, a regulatory subunit of the ligase IV complex specialized for NHEJ. This phosphorylation required cyclin-dependent kinase (CDK) activity as well as polo-like kinase 1 (Plk1). A phosphorylation-defective XRCC4 mutant showed more efficient M-phase DSB repair accompanied with an increase in anaphase bridge formation. These results suggest that phosphorylation of XRCC4 suppresses DSB repair by modulating ligase IV function to prevent genome instability during M phase. Taken together, our results indicate that XRCC4 is required not only for the promotion of NHEJ during interphase but also for its M-phase-specific suppression of DSB repair.

SETD2 is required for DNA double-strand break repair and activation of the p53-mediated checkpoint

Histone modifications establish the chromatin states that coordinate the DNA damage response. In this study, we show that SETD2, the enzyme that trimethylates histone H3 lysine 36 (H3K36me3), is required for ATM activation upon DNA double-strand breaks (DSBs). Moreover, we find that SETD2 is necessary for homologous recombination repair of DSBs by promoting the formation of RAD51 presynaptic filaments. In agreement, SETD2-mutant clear cell renal cell carcinoma (ccRCC) cells displayed impaired DNA damage signaling. However, despite the persistence of DNA lesions, SETD2-deficient cells failed to activate p53, a master guardian of the genome rarely mutated in ccRCC and showed decreased cell survival after DNA damage. We propose that this novel SETD2-dependent role provides a chromatin bookmarking instrument that facilitates signaling and repair of DSBs. In ccRCC, loss of SETD2 may afford an alternative mechanism for the inactivation of the p53-mediated checkpoint without the need for additional genetic mutations in TP53.

DOI:http://dx.doi.org/10.7554/eLife.02482.001

Normal wear and tear, exposure to chemicals, and ultraviolet light can all damage DNA, so cells rely on a range of sensors and mechanisms to detect and repair damaged DNA. Cells also package DNA molecules inside structures called histones to protect them against damage.

Double-strand breaks—one of the most serious forms of DNA damage—are detected by an enzyme called ATM, and can be repaired in two ways. Bringing the broken strands back together is an obvious method, but it is also error prone. Using templates to generate new DNA to repair the damage is less prone to error, but it can only happen at certain times of the cell cycle.

Some cancers are linked to the faulty repair of double-strand breaks. Moreover, a type of kidney cancer called clear cell renal carcinoma is linked to a lack of activity by a protein called p53, even in individuals who don't have mutations in the gene for this protein. However, many people with this type of cancer have mutations in the gene for a protein called SETD2.

To investigate the links between SETD2 and DNA repair, Carvalho et al. compared cells with and without mutations in the gene for SETD2. It emerged that SETD2 must be present for DNA repair to take place: the SETD2 modifies the histones so that they can recruit the enzymes that repair the DNA via the template approach (which is relatively error free). SETD2 may be particularly important for repairing damage to genes without introducing errors.

Carvalho et al. also show that mutations in SETD2 are sufficient to inactivate p53. The gene for this protein, which impedes the proliferation of cells with genomic aberrations, such as double-strand breaks, is mutated in most cancers. Overall the results help to illustrate how histone modifications and the DNA damage repair mechanisms and checkpoints work in concert to suppress cancer.

DOI:http://dx.doi.org/10.7554/eLife.02482.002

Oxidatively induced DNA damage: mechanisms, repair and disease

Endogenous and exogenous sources cause oxidatively induced DNA damage in living organisms by a variety of mechanisms. The resulting DNA lesions are mutagenic and, unless repaired, lead to a variety of mutations and consequently to genetic instability, which is a hallmark of cancer. Oxidatively induced DNA damage is repaired in living cells by different pathways that involve a large number of proteins. Unrepaired and accumulated DNA lesions may lead to disease processes including carcinogenesis. Mutations also occur in DNA repair genes, destabilizing the DNA repair system. A majority of cancer cell lines have somatic mutations in their DNA repair genes. In addition, polymorphisms in these genes constitute a risk factor for cancer. In general, defects in DNA repair are associated with cancer. Numerous DNA repair enzymes exist that possess different, but sometimes overlapping substrate specificities for removal of oxidatively induced DNA lesions. In addition to the role of DNA repair in carcinogenesis, recent evidence suggests that some types of tumors possess increased DNA repair capacity that may lead to therapy resistance. DNA repair pathways are drug targets to develop DNA repair inhibitors to increase the efficacy of cancer therapy. Oxidatively induced DNA lesions and DNA repair proteins may serve as potential biomarkers for early detection, cancer risk assessment, prognosis and for monitoring therapy. Taken together, a large body of accumulated evidence suggests that oxidatively induced DNA damage and its repair are important factors in the development of human cancers. Thus this field deserves more research to contribute to the development of cancer biomarkers, DNA repair inhibitors and treatment approaches to better understand and fight cancer.

DNA damage induces reactive oxygen species generation through the H2AX-Nox1/Rac1 pathway

The DNA damage response (DDR) cascade and ROS (reactive oxygen species) signaling are both involved in the induction of cell death after DNA damage, but a mechanistic link between these two pathways has not been clearly elucidated. This study demonstrates that ROS induction after treatment of cells with neocarzinostatin (NCS), an ionizing radiation mimetic, is at least partly mediated by increasing histone H2AX. Increased levels of ROS and cell death induced by H2AX overexpression alone or DNA damage leading to H2AX accumulation are reduced by treating cells with the antioxidant N-Acetyl-L-Cysteine (NAC), the NADP(H) oxidase (Nox) inhibitor DPI, expression of Rac1N17, and knockdown of Nox1, but not Nox4, indicating that induction of ROS by H2AX is mediated through Nox1 and Rac1 GTPase. H2AX increases Nox1 activity partly by reducing the interaction between a Nox1 activator NOXA1 and its inhibitor 14-3-3zeta. These results point to a novel role of histone H2AX that regulates Nox1-mediated ROS generation after DNA damage.

Regiospecific O-methylation of naphthoic acids catalyzed by NcsB1, an O-methyltransferase involved in the biosynthesis of the enediyne antitumor antibiotic neocarzinostatin

Neocarzinostatin, a clinical anticancer drug, is the archetypal member of the chromoprotein family of enediyne antitumor antibiotics that are composed of a nonprotein chromophore and an apoprotein. The neocarzinostatin chromophore consists of a nine-membered enediyne core, a deoxyaminosugar, and a naphthoic acid moiety. We have previously cloned and sequenced the neocarzinostatin biosynthetic gene cluster and proposed that the biosynthesis of the naphthoic acid moiety and its incorporation into the neocarzinostatin chromophore are catalyzed by five enzymes NcsB, NcsB1, NcsB2, NcsB3, and NcsB4. Here we report the biochemical characterization of NcsB1, unveiling that: (i) NcsB1 is an S-adenosyl-L-methionine-dependent O-methyltransferase; (ii) NcsB1 catalyzes regiospecific methylation at the 7-hydroxy group of its native substrate, 2,7-dihydroxy-5-methyl-1-naphthoic acid; (iii) NcsB1 also recognizes other dihydroxynaphthoic acids as substrates and catalyzes regiospecific O-methylation; and (iv) the carboxylate and its ortho-hydroxy groups of the substrate appear to be crucial for NcsB1 substrate recognition and binding, and O-methylation takes place only at the free hydroxy group of these dihydroxynaphthoic acids. These findings establish that NcsB1 catalyzes the third step in the biosynthesis of the naphthoic acid moiety of the neocarzinostatin chromophore and further support the early proposal for the biosynthesis of the naphthoic acid and its incorporation into the neocarzinostatin chromophore with free naphthoic acids serving as intermediates. NcsB1 represents another opportunity that can now be exploited to produce novel neocarzinostatin analogs by engineering neocarzinostatin biosynthesis or applying directed biosynthesis strategies.

Protein–Small Molecule Interactions in Neocarzinostatin, the Prototypical Enediyne Chromoprotein Antibiotic

The enediyne chromoproteins are a class of potent antitumour antibiotics comprising a 1:1 complex of a protein and a noncovalently bound chromophore. The protein is required to protect and transport the highly labile chromophore, which acts as the cytotoxic component by reacting with DNA leading to strand cleavage. A derivative of the best-studied member of this class, neocarzinostatin (NCS), is currently in use as a chemotherapeutic in Japan. The application of the chromoproteins as therapeutics along with their unique mode of action has prompted widespread interest in this area. Notable developments include the discovery of non-natural ligands for the apoproteins and the observation that multiple binding modes are available for these ligands in the binding site. Mutation studies on the apoproteins have revealed much about their stability and variability, and the application of an in vitro evolution method has conferred new binding specificity for unrelated ligands. These investigations hold great promise for the application of the apoproteins for drug-delivery, transport and stabilisation systems.

Condensin I recruitment and uneven chromatin condensation precede mitotic cell death in response to DNA damage

Mitotic cell death (MCD) is a prominent but poorly defined form of death that stems from aberrant mitosis. One of the early steps in MCD is premature mitosis and uneven chromatin condensation (UCC). The mechanism underlying this phenomenon is currently unknown. In this study, we show that DNA damage in cells with a compromised p53-mediated G2/M checkpoint triggers the unscheduled activation of cyclin-dependent kinase 1 (Cdk1), activation and chromatin loading of the condensin I complex, and UCC followed by the appearance of multimicronucleated cells, which is evidence of MCD. We demonstrate that these processes engage some of the players of normal mitotic chromatin packaging but not those that drive the apoptotic chromatin condensation. Our findings establish a link between the induction of DNA damage and mitotic abnormalities (UCC) through the unscheduled activation of Cdk1 and recruitment of condensin I. These results demonstrate a clear distinction between the mechanisms that drive MCD-associated and apoptosis-related chromatin condensation and provide mechanistic insights and new readouts for a major cell death process in treated tumors.

Oxidative DNA damage and disease: induction, repair and significance

The generation of reactive oxygen species may be both beneficial to cells, performing a function in inter- and intracellular signalling, and detrimental, modifying cellular biomolecules, accumulation of which has been associated with numerous diseases. Of the molecules subject to oxidative modification, DNA has received the greatest attention, with biomarkers of exposure and effect closest to validation. Despite nearly a quarter of a century of study, and a large number of base- and sugar-derived DNA lesions having been identified, the majority of studies have focussed upon the guanine modification, 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-OH-dG). For the most part, the biological significance of other lesions has not, as yet, been investigated. In contrast, the description and characterisation of enzyme systems responsible for repairing oxidative DNA base damage is growing rapidly, being the subject of intense study. However, there remain notable gaps in our knowledge of which repair proteins remove which lesions, plus, as more lesions identified, new processes/substrates need to be determined. There are many reports describing elevated levels of oxidatively modified DNA lesions, in various biological matrices, in a plethora of diseases; however, for the majority of these the association could merely be coincidental, and more detailed studies are required. Nevertheless, even based simply upon reports of studies investigating the potential role of 8-OH-dG in disease, the weight of evidence strongly suggests a link between such damage and the pathogenesis of disease. However, exact roles remain to be elucidated.

Refined solution structure of a DNA heteroduplex containing an aldehydic abasic site

The solution structure of the DNA duplex d(C1G2C3G4A5D6A7C8G9C10C11)-d (G22C21G20C19T18A17T16G15C14G13-G12), with D indicating a deoxyribose aldehyde abasic site and numbering from 5' to 3', has been determined by the combined use of NMR and restrained molecular dynamics. The 31P and 31P-1H correlation data indicate that the backbones of these duplex DNAs are regular. One- and two-dimensional 1H NMR data indicate that the duplexes are right-handed and B-form. Conformational changes due to the presence of the abasic site extends to the base pairs adjacent to the lesion site with the local conformation of the DNA being dependent on whether the abasic site is in the alpha or beta configuration. When the sugar of the abasic site is in the beta configuration the deoxyribose is within the helix, whereas when the sugar is in the alpha configuration the deoxyribose is out of the helix. The base of residue A17 in the position opposite the abasic site is predominantly stacked in the helix in both cases. A water molecule can apparently form a hydrogen bond bridge between the beta abasic site and A17.

Antitumor resistance induced by zinostatin stimalamer (ZSS), a polymer-conjugated neocarzinostatin (NCS) derivative. I. Meth A tumor eradication and tumor-neutralizing activity in mice pretreated with ZSS or NCS

Zinostatin stimalamer (ZSS) is a new anticancer agent derived from neocarzinostatin (NCS), which is synthesized by conjugation of one molecule of NCS and two molecules of poly(styrene-co-maleic acid). ZSS exhibited potent in vitro and in vivo antitumor activity in preclinical experiments, and a clinical trial of the intra-arterial administration of ZSS with iodized oil on hepatocellular carcinoma showed potent antitumor activity. We investigated the effect of ZSS and NCS on antitumor resistance and found that pretreatment with either drug suppressed the growth of MethA tumors in Balb/c mice and induced tumor eradication when given separately by single administration at therapeutic doses between 1 day and 4 weeks before tumor transplantation. The findings that the cytocidal activity of these drugs was not detected in vivo at the time of tumor transplantation and that tumor regression was preceded by a period of transient growth suggested that tumor regression was due to host-mediated antitumor activity induced by these drugs. Pretreatment with ZSS or NCS also suppressed the growth of Colon 26 carcinoma and Sarcoma 180. The finding that NCS showed the same effect as ZSS suggests that poly(styrene-comaleic acid) is not essential for the induction of host-mediated antitumor activity. Furthermore, apo-ZSS, which lacks cytocidal activity, did not induce antitumor activity. From this, it is suggested that the cytocidal effect of ZSS involves the induction of host-mediated antitumor resistance. In athymic Balb/c nu/nu mice, pretreatment with ZSS or NCS did not induce tumor eradication, suggesting that mature T lymphocytes play an important role in tumor eradication. Challenging MethA was rejected without transient growth in mice that had been cured of MethA, but challenging Colon 26 was not, showing that anti-MethA resistance was augmented selectively in the MethA-eradicated mice. Splenocytes from MethA-bearing mice pretreated with the drug showed tumor-neutralizing activity beginning 14 days after tumor transplantation. Tumor-neutralizing activity was only induced after MethA transplantation. The effector cells of this tumor-neutralizing activity were Thy1.2+ T lymphocytes that had been passed through a nylon-wool column, but no significant augmentation of cell-mediated cytotoxic activity of splenocytes from MethA-eradicated mice was observed in vitro.

Enzymic- and thiol-mediated activation of halogen-substituted diaziridinylbenzoquinones: redox transitions of the semiquinone and semiquinone-thioether species

Activation of 2,5-diaziridinyl-1,4-benzoquinones bearing halogen (Cl, Br, or F) substituents at C3 and C6 by NADPH-cytochrome P450 reductase and glutathione nucleophilic substitution was examined in terms of free radical production and DNA strand scission. A semiquinone species was observed by direct ESR in aerobic conditions during: (a) NADPH-cytochrome P450 reductase-catalyzed reduction of the above quinones. (b) The interaction of these quinones with GSH entailing primarily reactivity of halogen substituents toward sulfur substitution. (c) NADPH-cytochrome P450 reductase-catalyzed activation of products resulting from the quinone/GSH interaction. The semiquinone ESR signal observed during enzymic catalysis was suppressed by superoxide dismutase and was not affected by catalase. ESR studies in conjunction with the spin trapping technique on the autoxidation of the semiquinones formed by the above reaction pathways indicated the formation of superoxide radicals. In addition, thiyl radicals were formed during the reactions following glutathione necleophilic substitution of the above quinones. The ESR signals of both superoxide and thiyl radicals were abolished by superoxide dismutase. No hydroxyl radicals were formed in solution during the redox transitions of these halogen-containing diaziridinylbenzoquinones. Bioreductive activation of these compounds via NADPH-cytochrome P450 reductase or sulfur nucleophilic substitution was associated with the formation of DNA strand breaks. This process was substantially inhibited (74-86%) by superoxide dismutase and to a lesser extent (23-31%) by catalase. It is suggested that DNA strand breakage proceeds in a manner entailing a semiquinone-dependent reduction of metal-ligands bound at the DNA surface and leading to site-specific, hydroxyl radical production.

One- and two-electron reduction of 2-methyl-1,4-naphthoquinone bioreductive alkylating agents: kinetic studies, free-radical production, thiol oxidation and DNA-strand-break formation

The one- and two-electron enzymic reduction of the bioreductive alkylating agents 2-methylmethoxynaphthoquinone (quinone I) and 2-chloromethylnaphthoquinone (quinone II) was studied with purified NADPH-cytochrome P-450 reductase and DT-diaphorase respectively, and characterized in terms of kinetic constants, oxyradical production, thiol oxidation and DNA-strand-break formation. The catalytic-centre activity values indicated that DT-diaphorase catalysed the reduction of quinone I far more efficiently than NADPH-cytochrome P-450 reductase, although the Km values of the two enzymes for this quinone were similar (1.2-3.0 microM). The one-electron-transfer flavoenzyme also catalysed the reduction of quinone II, but the behaviour of DT-diaphorase towards this quinone did not permit calculation of kinetic constants. A salient feature of the redox transitions caused by the one- and two-electron catalysis of these quinones was the different contributions of disproportionation and autoxidation reactions respectively. In the former case, about 26% of NADPH consumed was accounted for in terms of autoxidation (as H2O2 formation), whereas in the latter, the autoxidation component accounted for most (98%) of the NADPH consumed. This difference was abrogated by superoxide dismutase, which enhanced autoxidation during NADPH-cytochrome P-450 catalysis to a maximal value. E.s.r. analysis indicated the formation of superoxide radicals, the signal of which was suppressed by superoxide dismutase and unaffected by catalase. The one- and two-electron reduction of these quinones in the presence of GSH was accompanied by formation of thiyl radicals. Although superoxide dismutase suppressed the thiol radical e.s.r. signal in both instances, the enzyme enhanced GSSG accumulation during NADPH-cytochrome P-450 catalysis of quinone I, whereas it inhibited GSSG formation during reduction of the quinone by DT-diaphorase. One- and two-electron reduction of quinone I led to calf thymus DNA-strand-break formation, a process that (a) was substantially decreased in experiments performed with dialysed DNA and in the presence of desferal and (b) was partially sensitive to superoxide dismutase and/or catalase. These findings are rationalized in terms of the occurrence of metal ions ligated to DNA, protecting against the toxic effects of superoxide radicals generated during enzymic reduction of quinones.

Chemistry of drug-induced DNA lesions

Recent results on the action mechanisms of naturally occurring DNA-damaging antitumor antibiotics have been described. These antibiotics include neocarzinostatin (NCS) and DNA alkylating, duocarmycin A and kapurimycin A3. A series of duplex hexanucleotides of modified bases were prepared and their selectivity for C5' and C4' oxidation in the NCS-mediated degradation was investigated. Based on the cleavage data, a new binding model that permits competitive hydrogen abstraction from C5' and C4' of the DNA deoxyribose moiety has been described. Chemistry of alkylation of self complementary octanucleotide d(CGTATACG)2 by antitumor antibiotic duocarmycin A was described. It was demonstrated that N3 of adenine6 attacks the cyclopropane subunit of duocarmycin A to produce the covalently alkylated adduct. In contrast, antibiotic kapurimycin A3 alkylate N7 of guanine4 of d(CGCG)2 to provide the corresponding covalent adduct. Heating at 90 degrees C degraded the adduct to kapurimycin A3-guanine adduct and the respective abasic site-containing oligonucleotide. The structures of heat-induced abasic sites were unambiguously characterized.

Kedarcidin chromophore: an enediyne that cleaves DNA in a sequence-specific manner

Kedarcidin chromophore is a 9-membered enediyne, recently isolated from an actinomycete strain. In vivo studies show this molecule to be extremely active against P388 leukemia and B16 melanoma. Cytotoxicity assays on the HCT116 colon carcinoma cell line result in an IC50 value of 1 nM. In vitro experiments with phi X174, pM2 DNA, and 32P-end-labeled restriction fragments demonstrate that this chromophore binds and cleaves duplex DNA with a remarkable sequence selectivity producing single-strand breaks. The cleavage chemistry requires reducing agents and oxygen similar to the other naturally occurring enediynes. Certain cations (Ca2+ and Mg2+) prevent strand cleavage. High-resolution 1H NMR studies on the chromophore in the presence of calcium chloride implicate the 2-hydroxynaphthoyl moiety in DNA binding. Interestingly, the kedarcidin chromophore appears structurally related to neocarzinostatin yet recognizes specific DNA sequences in a manner similar to calicheamicin gamma 1I, an enediyne with a significantly different structure. Moreover, kedarcidin and calicheamicin share a DNA preferred site, the TCCTN-mer. These observations indicate that the individual structural features of these agents are not solely responsible for their DNA selectivity. Rather, a complementarity between their overall tertiary structure and the local conformation of the DNA at the binding sites must play a significant role in the recognition process.

Analysis of DNA fragmentation in human uterine cervix carcinoma HeLa S3 cells treated with duocarmycins or other antitumor agents by pulse field gel electrophoresis

Pulse field gel electrophoresis using a contour-clamped homogeneous electric field was applied for the analysis of DNA-fragmenting activity of antitumor agents towards human uterine cervix carcinoma HeLa S3 cells. Duocarmycins (DUMs), novel antitumor antibiotics with ultrapotent cell growth-inhibitory activities, caused DNA fragmentation at 10 times their IC50 values at 2 h exposure. At 100 times their IC50 values, the size of the smallest fragments was about 245 kilobase pairs (kbp). DUMA, DUMB1 and DUMB2 exhibited similar DNA fragmentation patterns, suggesting similar action mechanisms. DNA fragmentation was also detected in cells treated with radical producers, intercalators and topoisomerase inhibitors. Two bands of about 1800 and 1500 kbp were commonly detected in the cells treated with DUMs and these agents. In addition, fragments of about 900 kbp were detected in the cells treated with a topoisomerase inhibitor, 4'-(9-acridinylamino)methane-sulfon-m-anisidine, and fragments in the broad size range between 700 and 245 kbp in the cells treated with radical producers, bleomycin and neocarzinostatin. DUMs showed a characteristic DNA fragmentation pattern, since both types of fragments induced by the topoisomerase inhibitor and the radical producers were simultaneously detected, suggesting a novel mode of interaction with DNA. DNA-crosslinking agents and mitotic inhibitors did not induce DNA fragmentation under these conditions. The pulse field gel electrophoresis is potentially useful for characterizing DNA-cleaving activity of various antitumor agents at the cellular level.

Nitroxide-mediated protection against X-ray- and neocarzinostatin-induced DNA damage

The stable free radical Tempol (4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy) has been shown to protect against X-ray-induced cytotoxicity and hydrogen peroxide- or xanthine oxidase-induced cytotoxicity and mutagenicity. The ability of Tempol to protect against X-ray- or neocarzinostatin (NCS)-induced mutagenicity or DNA double-strand breaks (dsb) was studied in Chinese hamster cells. Tempol (50 mM) provided a protection factor of 2.7 against X-ray-induced mutagenicity in Chinese hamster ovary (CHO) AS52 cells, with a protection factor against cytotoxicity of 3.5. Using the field inversion gel electrophoresis technique of measuring DNA dsb, 50 mM Tempol provides a threefold reduction in DNA damage at an X-ray dose of 40 Gy. For NCS-induced damage, Tempol increased survival from 9% to 80% at 60 ng/mL NCS and reduced mutation induction by a factor of approximately 3. DNA dsb were reduced by a factor of approximately 7 at 500 ng/mL NCS. Tempol is representative of a class of stable nitroxide free radical compounds that have superoxide dismutase-mimetic activity, can oxidize metal ions such as ferrous iron that are complexed to DNA, and may also detoxify radiation-induced organoperoxide radicals by competitive scvenging. The NCS chromophore is reduced by sulfhydryls to an active form. Electron spin resonance (ESR) spectroscopy shows that 2-mercaptoethanol-activated NCS reacts with Tempol 3.5 times faster than does unactivated NCS. Thus, Tempol appears to inactivate the NCS chromophore before a substantial amount of DNA damage occurs.

C-1' hydrogen abstraction of deoxyribose in DNA strand scission by dynemicin A

Dynemicin A, which is a hybrid antitumor antibiotic containing anthraquinone and enediyne cores, abstracts the C-1' hydrogen of DNA deoxyribose and then the damaged DNA leads to strand breaks with the formation of 5'- and 3'-phosphate termini. The lesions of C-4' hydrogen also occur at 3' side of G.C base pairs (i. e., 5'-CT and 5'-GA), leading to 5'-phosphate and 3'-phosphoglycolate termini or 4'-hydroxylated abasic sites. The C-1' hydrogen abstraction by dynemicin A is distinct from the preferential C-5' hydrogen abstraction of calicheamicin and neocarzinostatin.

Activation of DNA cleavage by dynemicin A in a B-Z conformational junction

We report here that the DNA strand scission by dynemicin A is not only sequence-specific but also conformation-specific. The salt-induced B----Z conformational transition dramatically enhanced the cleavage by dynemicin A in a B-Z junction region. By contrast, the bleomycin-Fe(II) complex, the elsamicin A-Fe(II) complex, and esperamicin A1 did not induce any preferential DNA cutting in such a DNA structure. The characteristic hyperreactivity of dynemicin A is observed in (dC-dG)8- and (dC-dG)12-inserted DNAs, but not in (dC-dG)5-inserted DNA. These results suggest value in the use of dynemicin A as proof of the existence of a B-Z junction in vivo and also may aid in understanding the structure of B-Z junctions.

Preferential hydroxylation by the chemical nuclease meso-tetrakis-(4-N-methylpyridiniumyl)porphyrinatomanganeseIII pentaacetate/KHSO5 at the 5' carbon of deoxyriboses on both 3' sides of three contiguous A.T base pairs in short double-stranded oligonucleotides

Selected double-stranded oligodeoxyribonucleotides have been used to probe, at the molecular level, DNA chain breakages induced by the chemical nuclease mesotetrakis(4-N- methylpyridiniumyl)porphyrinatomanganeseIII pentaacetate/KHSO5. The results show that cleavage selectively occurs on the two 3' sides of three contiguous A.T base pairs (an A.T triplet). Hydroxylation at 5' carbon of the deoxyribose targets represents the initial damage on the sugar-phosphodiester backbone and leaves a 3' phosphate and a 5' aldehyde at the ends. The fragments were separated by HPLC and unambiguously identified through chemical and biochemical reactions and/or sequencing after enzymatic conversion to mononucleosides. Also studied was the degradation of a 22-nucleotide DNA molecule containing two A.T triplets. Gel electrophoresis analyses on the corresponding 5'-32P-end-labeled substrate supported the above cleavage specificity and mechanism.

Characterization of conformational features of DNA heteroduplexes containing aldehydic abasic sites

The DNA duplexes shown below, with D indicating deoxyribose aldehyde absic sites and numbering from 5' to 3', have been investigated by NMR. The 31P and 31P-1H correlation data indicate [formula: see text] that the backbones of these duplex DNAs are regular. One- and two-dimensional 1H NMR data indicate that the duplexes are right-handed and B-form. Conformational changes due to the presence of the abasic site extend to the two base pairs adjacent to the lesion site with the local conformation of the DNA being dependent on whether the abasic site is in the alpha or beta configuration. The aromatic base of residue A17 in the position opposite the abasic site is predominantly stacked in the helix as is G17 in the analogous sample. Imino lifetimes of the AT base pairs are much longer in samples with an abasic site than in those containing a Watson-Crick base pair. The conformational and dynamical properties of the duplex DNAs containing the naturally occurring aldehyde abasic site are different from those of duplex DNAs containing a variety of analogues of the abasic site.

Selective abstraction of 2H from C-5' of thymidylate in an oligodeoxynucleotide by the radical center at C-6 of the diradical species of neocarzinostatin: chemical evidence for the structure of the activated drug-DNA complex

Use has been made of the mechanism of DNA deoxyribose damage by the ene-diyne-containing chromophore of the antitumor antibiotic neocarzinostatin to provide chemical evidence for the structure of the activated drug-DNA complex. Radical centers at C-2 and C-6 of the diradical form of the glutathione-activated chromophore abstract hydrogen atoms from C-1' of the C residue and C-5' of the T residue in AGC.GCT to generate a bistranded lesion consisting of an abasic site at C and a strand break at T. This laboratory has proposed a molecular model for the drug-DNA interaction in which the naphthoate moiety of the chromophore intercalates between A.T and G.C, placing the diradical core in the minor groove, so that the radical centers at C-6 and C-2 are close to C-5' of T and C-1' of C, respectively. To determine which radical center abstracts one of the hydrogen atoms from C-5', the self-complementary oligodeoxynucleotide GCAGCGCTGC was synthesized with 2H at both 5' positions of the T residue and treated with glutathione-activated chromophore. Sequencing-gel electrophoresis showed that drug attack was limited to the T and C residues and that abstraction of 2H from C-5' exhibited a small isotope selection effect of 1.25. 1H NMR spectroscopic examination of the reacted chromophore, isolated by HPLC, indicated that 2H was selectively abstracted by C-6, providing experimental corroboration of the model and further elucidating the chemical mechanism. Since direct strand breakage at the T residue exceeds (44% more) abasic site formation at the C residue, other models of drug-DNA interaction leading to only single-strand breaks are also considered.

Enhancement by verapamil of neocarzinostatin action on multidrug-resistant Chinese hamster ovary cells: possible release of nonprotein chromophore in cells

Multidrug-resistant CHRC5 cells were about 10-fold more resistant to the proteinaceous anticancer drug neocarzinostatin (NCS) and its nonprotein chromophore (NPC) than the parental AUXB1 cells. There was little difference in cell growth, glutathione content, or activities of several antioxidant enzymes between the two cell lines. The degree of intracellular incorporation and extracellular excretion of fluorescein isothiocyanate-labeled NCS by CHRC5 cells was similar to that of AUXB1 cells. On the other hand, 20 microM verapamil or 27 microM cepharanthine restored the susceptibility of CHRC5 cells to NCS and NPC to the level of AUXB1 cells. In addition, NPC was found to suppress the photolabeling of [3H]azidopine (a known P-glycoprotein-binding ligand) to plasma membranes of CHRC5 cells. All these findings favor the possibility that NPC was excreted via P-glycoprotein, which may contribute to the resistance of CHRC5 cells to NCS.

DNA intercalation and cleavage of an antitumor antibiotic dynemicin that contains anthracycline and enediyne cores

Dynemicin is a hybrid containing anthraquinone and enediyne cores, which contribute to binding and cleavage of DNA, respectively. DNA strand scission by the antitumor antibiotic is significantly enhanced by the addition of NADPH or thiol compounds. The preferential cutting site of dynemicin is on the 3' side of purine bases (i.e., 5'-GC, -GT, and -AG) and is clearly different from the cutting sites of esperamicin and calicheamicin. The double-stranded and the stem regions of single-stranded DNAs are preferentially cleaved by dynemicin. Therefore, dynemicin may be a useful reagent for probing secondary structures of DNA. Pretreatment of DNA with Adriamycin and actinomycin D alters the cutting mode of dynemicin. Dynemicin-mediated DNA breakage is strongly inhibited by pretreatment of the DNA with distamycin A and anthramycin, suggesting that dynemicin interacts with the minor groove of the DNA helix. Intercalation of the anthraquinone core into the DNA followed by the attack of the phenyl diradical formed from the enediyne core is considered as a possible mechanism of action of dynemicin.

Molecular models of neocarzinostatin damage of DNA: analysis of sequence dependence in 5'GAGCG:5'CGCTC

Model building and molecular mechanics and dynamics calculations have been performed on a number of complexes of the post-activated form of the neocarzinostatin chromophore (NCS) with the B-DNA oligomer 5'GAGCG:5'CGCTC. Stable structures with the naphthoic acid moiety intercalated at all base pairs can be constructed. The observed bistranded lesions consisting of an abasic site at the Cyt residue in AGC and a direct break at the Thy residue on the complementary strand can be explained by assuming that NCS in the (R,R) form intercalates between the Ade2-Thy9/Gua3-Cyt8 base step with its 'diradical' core oriented towards the 3'-end of the (+) strand. Sites at C5', C4' and C1' in the minor groove are within a short enough distance from the two radical centers on NCS to permit hydrogen atom abstraction and the formation of the bistranded lesions. Strand cleavage at Thy9 may occur as a single lesion if NCS is intercalated into the Gua3-Cyt8/Cyt4-Gua7 base step with its active core towards the 3'-end of the (-) strand. The results are analyzed, and the utility and limitations of this type of model building are discussed.