Single-strand and double-strand deoxyribonucleic acid breaks produced by several bleomycin analogues

Alpha-synuclein modulates the repair of genomic DNA double-strand breaks in a DNA-PKcs-regulated manner

α-synuclein (αSyn) is a presynaptic and nuclear protein that aggregates in important neurodegenerative diseases such as Parkinson's Disease (PD), Parkinson's Disease Dementia (PDD) and Lewy Body Dementia (LBD). Our past work suggests that nuclear αSyn may regulate forms of DNA double-strand break (DSB) repair in HAP1 cells after DNA damage induction with the chemotherapeutic agent bleomycin1. Here, we report that genetic deletion of αSyn specifically impairs the non-homologous end-joining (NHEJ) pathway of DSB repair using an extrachromosomal plasmid-based repair assay in HAP1 cells. Notably, induction of a single DSB at a precise genomic location using a CRISPR/Cas9 lentiviral approach also showed the importance of αSyn in regulating NHEJ in HAP1 cells and primary mouse cortical neuron cultures. This modulation of DSB repair is regulated by the activity of the DNA damage response signaling kinase DNA-PKcs, since the effect of αSyn loss-of-function is reversed by DNA-PKcs inhibition. Together, these findings suggest that αSyn plays an important physiologic role in regulating DSB repair in both a transformed cell line and in primary cortical neurons. Loss of this nuclear function may contribute to the neuronal genomic instability detected in PD, PDD and LBD and points to DNA-PKcs as a potential therapeutic target.

Alpha-synuclein regulates the repair of genomic DNA double-strand breaks in a DNA-PKcs-dependent manner

α-synuclein (αSyn) is a presynaptic and nuclear protein that aggregates in important neurodegenerative diseases such as Parkinson's Disease (PD), Parkinson's Disease Dementia (PDD) and Lewy Body Dementia (LBD). Our past work suggests that nuclear αSyn may regulate forms of DNA double-strand break (DSB) repair in HAP1 cells after DNA damage induction with the chemotherapeutic agent bleomycin1. Here, we report that genetic deletion of αSyn specifically impairs the non-homologous end-joining (NHEJ) pathway of DSB repair using an extrachromosomal plasmid-based repair assay in HAP1 cells. Importantly, induction of a single DSB at a precise genomic location using a CRISPR/Cas9 lentiviral approach also showed the importance of αSyn in regulating NHEJ in HAP1 cells and primary mouse cortical neuron cultures. This modulation of DSB repair is dependent on the activity of the DNA damage response signaling kinase DNA-PKcs, since the effect of αSyn loss-of-function is reversed by DNA-PKcs inhibition. Using in vivo multiphoton imaging in mouse cortex after induction of αSyn pathology, we find an increase in longitudinal cell survival of inclusion-bearing neurons after Polo-like kinase (PLK) inhibition, which is associated with an increase in the amount of aggregated αSyn within inclusions. Together, these findings suggest that αSyn plays an important physiologic role in regulating DSB repair in both a transformed cell line and in primary cortical neurons. Loss of this nuclear function may contribute to the neuronal genomic instability detected in PD, PDD and DLB and points to DNA-PKcs and PLK as potential therapeutic targets.

Pulmonary Delivery of Recombinant Human Bleomycin Hydrolase Using Mannose-Modified Hierarchically Porous UiO-66 for Preventing Bleomycin-Induced Pulmonary Fibrosis

Bleomycins (BLMs) are widely used in clinics as antitumor agents. However, BLM-based chemotherapies often accompany severe pulmonary fibrosis (PF). Human bleomycin hydrolase is a cysteine protease that can convert BLMs into inactive deamido-BLMs. In this study, mannose-modified hierarchically porous UiO-66 (MHP-UiO-66) nanoparticles (NPs) were used to encapsulate the recombinant human bleomycin hydrolase (rhBLMH). When rhBLMH@MHP-UiO-66 was intratracheally instilled into the lungs, the NPs were transported into the epithelial cells, and rhBLMH prevented the lungs from PF during BLM-based chemotherapies. Encapsulation of rhBLMH in the MHP-UiO-66 NPs protects the enzyme from proteolysis in physiological conditions and enhances cellular uptake. In addition, the MHP-UiO-66 NPs significantly enhance the pulmonary accumulation of intratracheally instilled rhBLMH, thus providing more efficient protection of the lungs against BLMs during the chemotherapies.

TEB/POLQ plays dual roles in protecting Arabidopsis from NO-induced DNA damage

Nitric oxide (NO) is a key player in numerous physiological processes. Excessive NO induces DNA damage, but how plants respond to this damage remains unclear. We screened and identified an Arabidopsis NO hypersensitive mutant and found it to be allelic to TEBICHI/POLQ, encoding DNA polymerase θ. The teb mutant plants were preferentially sensitive to NO- and its derivative peroxynitrite-induced DNA damage and subsequent double-strand breaks (DSBs). Inactivation of TEB caused the accumulation of spontaneous DSBs largely attributed to endogenous NO and was synergistic to DSB repair pathway mutations with respect to growth. These effects were manifested in the presence of NO-inducing agents and relieved by NO scavengers. NO induced G2/M cell cycle arrest in the teb mutant, indicative of stalled replication forks. Genetic analyses indicate that Polθ is required for translesion DNA synthesis across NO-induced lesions, but not oxidation-induced lesions. Whole-genome sequencing revealed that Polθ bypasses NO-induced base adducts in an error-free manner and generates mutations characteristic of Polθ-mediated end joining. Our experimental data collectively suggests that Polθ plays dual roles in protecting plants from NO-induced DNA damage. Since Polθ is conserved in higher eukaryotes, mammalian Polθ may also be required for balancing NO physiological signaling and genotoxicity.

SUPPRESSOR OF GAMMA RESPONSE 1 acts as a regulator coordinating crosstalk between DNA damage response and immune response in Arabidopsis thaliana

Plants live in constantly changing and often unfavorable or stressful environments. Environmental changes induce biotic and abiotic stress, which, in turn, may cause genomic DNA damage. Hence, plants simultaneously suffer abiotic/biotic stress and DNA damage. However, little information is available on the signaling crosstalk that occurs between DNA damage and abiotic/biotic stresses. Arabidopsis thaliana SUPPRESSOR OF GAMMA RESPONSE1 (SOG1) is a pivotal transcription factor that regulates thousands of genes in response to DNA double-strand break (DSB), and we recently reported that SOG1 has a role in immune responses. In the present study, the effects of SOG1 overexpression on the DNA damage and immune responses were examined. Results found that SOG1 overexpression enhances the regulation of numerous downstream genes. Relative to the wild type plants, then, DNA damage responses were observed to be strongly induced. SOG1 overexpression also upregulates chitin (a major components of fungal cell walls) responsive genes in the presence of DSBs, implying that pathogen defense response is activated by DNA damage via SOG1. Further, SOG1 overexpression enhances fungal resistance. These results suggest that SOG1 regulates crosstalk between DNA damage response and the immune response and that plants have evolved a sophisticated defense network to contend with environmental stress.

An Automated, Single Cell Quantitative Imaging Microscopy Approach to Assess Micronucleus Formation, Genotoxicity and Chromosome Instability

Micronuclei are small, extranuclear bodies that are distinct from the primary cell nucleus. Micronucleus formation is an aberrant event that suggests a history of genotoxic stress or chromosome mis-segregation events. Accordingly, assays evaluating micronucleus formation serve as useful tools within the fields of toxicology and oncology. Here, we describe a novel micronucleus formation assay that utilizes a high-throughput imaging platform and automated image analysis software for accurate detection and rapid quantification of micronuclei at the single cell level. We show that our image analysis parameters are capable of identifying dose-dependent increases in micronucleus formation within three distinct cell lines following treatment with two established genotoxic agents, etoposide or bleomycin. We further show that this assay detects micronuclei induced through silencing of the established chromosome instability gene, SMC1A. Thus, the micronucleus formation assay described here is a versatile and efficient alternative to more laborious cytological approaches, and greatly increases throughput, which will be particularly beneficial for large-scale chemical or genetic screens.

Polyamines alleviate the inhibitory effect of the DNA cross-linking agent mitomycin C on root growth

Polyamines (putrescine, spermidine and spermine) are ubiquitously present in various types of cells of living organisms. They are involved in a variety of cellular processes, including cell proliferation and cell differentiation, and are required for abiotic stress tolerances in plants. However, it is still not understood whether polyamines are involved in the plant growth inhibition caused by DNA-damaging agents. In this study, we examined the effects of polyamines on the inhibition of plant root growth and gene expression in Arabidopsis thaliana treated with mitomycin C (MMC), a genotoxic agent that induces DNA interstrand crosslinks. We found that polyamines alleviated the inhibitory effect caused by MMC on root growth. In addition, we also found that polyamines alleviated the increased expression of AtBRCA1 and AtRAD51 genes induced by MMC treatment. Our study provides the first evidence that polyamines contribute to tolerance against plant-growth inhibition caused by a DNA-damaging chemical.

Activities of recombinant human bleomycin hydrolase on bleomycins and engineered analogues revealing new opportunities to overcome bleomycin-induced pulmonary toxicity

The bleomycins (BLMs) are widely used in combination therapies for the treatment of various cancers. Dose-dependent and cumulative pulmonary toxicity is the major cause of BLM-associated morbidity, limiting the broad uses of BLMs as anticancer drugs. The organ specificity of BLM-induced toxicity has been correlated with the expression of the hBLMH gene, encoding the human bleomycin hydrolase (hBLMH), which is poorly expressed in the lung. hBLMH hydrolyzes BLMs into the biologically inactive deamido BLMs, thereby protecting organs from BLM-induced toxicity. Here we report (i) expression of hBLMH and production and isolation of recombinant human bleomycin hydrolase (rhBLMH) from E. coli, (ii) structural characterization of deamido BLM A2 and B2 isolated from rhBLMH-catalyzed hydrolysis of BLM A2 and B2, and (iii) kinetic characterization of the rhBLMH-catalyzed hydrolysis of BLM A2 and B2, in comparison with five BLM analogues. rhBLMH from E. coli catalyzes rapid and efficient hydrolysis of all BLMs tested, exhibiting a superior catalytic efficiency for BLM B2. These findings reveal new opportunities to overcome BLM-induced pulmonary toxicity in chemotherapies, potentially by exploring BLM B2 as the preferred congener, engineering designer BLMs with optimized activity for rhBLMH, or co-administrating rhBLMH directly into the lung as a potential protein therapeutic.

The Interaction of the Metallo-Glycopeptide Anti-Tumour Drug Bleomycin with DNA

The cancer chemotherapeutic drug, bleomycin, is clinically used to treat several neoplasms including testicular and ovarian cancers. Bleomycin is a metallo-glycopeptide antibiotic that requires a transition metal ion, usually Fe(II), for activity. In this review, the properties of bleomycin are examined, especially the interaction of bleomycin with DNA. A Fe(II)-bleomycin complex is capable of DNA cleavage and this process is thought to be the major determinant for the cytotoxicity of bleomycin. The DNA sequence specificity of bleomycin cleavage is found to at 5′-GT* and 5′-GC* dinucleotides (where * indicates the cleaved nucleotide). Using next-generation DNA sequencing, over 200 million double-strand breaks were analysed, and an expanded bleomycin sequence specificity was found to be 5′-RTGT*AY (where R is G or A and Y is T or C) in cellular DNA and 5′-TGT*AT in purified DNA. The different environment of cellular DNA compared to purified DNA was proposed to be responsible for the difference. A number of bleomycin analogues have been examined and their interaction with DNA is also discussed. In particular, the production of bleomycin analogues via genetic manipulation of the modular non-ribosomal peptide synthetases and polyketide synthases in the bleomycin gene cluster is reviewed. The prospects for the synthesis of bleomycin analogues with increased effectiveness as cancer chemotherapeutic agents is also explored.

F2-isoprostanes can mediate bleomycin-induced lung fibrosis

F₂-isoprostanes (F₂-IsoPs) have been considered markers of oxidative stress in various pulmonary diseases, but little is known about their possible role in pulmonary fibrosis. In this study, we have investigated the potential key role of F₂-IsoPs as markers and mediators of bleomycin (BLM)-induced pulmonary fibrosis in rats. During the in vivo study, plasma F₂-IsoPs showed a peak at 7 days and remained elevated for the entire experimental period. Lung F₂-IsoP content nearly tripled 7 days following the intratracheal instillation of BLM, and by 28 days, the value increased about fivefold compared to the controls. Collagen deposition correlated with F₂-IsoP content in the lung. Furthermore, from day 21 onwards, lung sections from BLM-treated animals showed α-smooth muscle actin (α-SMA) positive cells, which were mostly evident at 28 days. In vitro studies performed in rat lung fibroblasts (RLF) demonstrated that either BLM or F₂-IsoPs stimulated both cell proliferation and collagen synthesis. Moreover, RLF treated with F₂-IsoPs showed a significant increase of α-SMA expression compared to control, indicating that F₂-IsoPs can readily activate fibroblasts to myofibroblasts. Our data demonstrated that F₂-IsoPs can be mediators of key events for the onset and development of lung fibrosis, such as cell proliferation, collagen synthesis and fibroblast activation. Immunocytochemistry analysis, inhibition and binding studies demonstrated the presence of the thromboxane A₂ receptor (TP receptor) on lung fibroblasts and suggested that the observed effects may be elicited through the binding to this receptor. Our data added a new perspective on the role of F₂-IsoPs in lung fibrosis by providing evidence of a profibrotic role for these mediators in the pathogenesis of pulmonary fibrosis.

Animal Models of Pulmonary Fibrosis

Pulmonary fibrosis is a debilitating disease and is often fatal. It may be the consequence of direct lung injury or the result of genetic defects and occupational, environmental, or drug-related exposures. In many cases the etiology is unknown. The pathogenesis of all forms of pulmonary fibrosis regardless of type of injury or etiology is incompletely understood. These disorders are characterized by the accumulation of extracellular matrix in the lung interstitium with a loss of lung compliance and impaired gas exchange that ultimately leads to respiratory failure. Animal models of pulmonary fibrosis have become indispensable in the improved understanding of these disorders. Multiple models have been developed each with advantages and disadvantages. In this chapter we discuss the application of two of the most commonly employed direct lung instillation models, namely, the induction of pulmonary fibrosis with bleomycin or fluorescein isothiocyanate (FITC). We provide details on design, materials, and methods and describe how these models can be best undertaken. We also discuss methods to induce fibrosis in aged mice using murine gamma-herpesvirus (γHV-68) and approaches to exacerbate bleomycin- or FITC-induced fibrosis using γHV-68.

Non-canonical reader modules of BAZ1A promote recovery from DNA damage

Members of the ISWI family of chromatin remodelers mobilize nucleosomes to control DNA accessibility and, in some cases, are required for recovery from DNA damage. However, it remains poorly understood how the non-catalytic ISWI subunits BAZ1A and BAZ1B might contact chromatin to direct the ATPase SMARCA5. Here, we find that the plant homeodomain of BAZ1A, but not that of BAZ1B, has the unusual function of binding DNA. Furthermore, the BAZ1A bromodomain has a non-canonical gatekeeper residue and binds relatively weakly to acetylated histone peptides. Using CRISPR-Cas9-mediated genome editing we find that BAZ1A and BAZ1B each recruit SMARCA5 to sites of damaged chromatin and promote survival. Genetic engineering of structure-designed bromodomain and plant homeodomain mutants reveals that reader modules of BAZ1A and BAZ1B, even when non-standard, are critical for DNA damage recovery in part by regulating ISWI factors loading at DNA lesions and supporting transcriptional programs required for survival.

ISWI chromatin remodelers regulate DNA accessibility and have been implicated in DNA damage repair. Here, the authors uncover functions, in response to DNA damage, for the bromodomain of the ISWI subunit BAZ1B and for the non-canonical PHD and bromodomain modules of the paralog BAZ1A.

A novel class of chemicals that react with abasic sites in DNA and specifically kill B cell cancers

Most B cell cancers overexpress the enzyme activation-induced deaminase at high levels and this enzyme converts cytosines in DNA to uracil. The constitutive expression of this enzyme in these cells greatly increases the uracil content of their genomes. We show here that these genomes also contain high levels of abasic sites presumably created during the repair of uracils through base-excision repair. We further show that three alkoxyamines with an alkyne functional group covalently link to abasic sites in DNA and kill immortalized cell lines created from B cell lymphomas, but not other cancers. They also do not kill normal B cells. Treatment of cancer cells with one of these chemicals causes strand breaks, and the sensitivity of the cells to this chemical depends on the ability of the cells to go through the S phase. However, other alkoxyamines that also link to abasic sites- but lack the alkyne functionality- do not kill cells from B cell lymphomas. This shows that the ability of alkoxyamines to covalently link to abasic sites is insufficient for their cytotoxicity and that the alkyne functionality may play a role in it. These chemicals violate the commonly accepted bioorthogonality of alkynes and are attractive prototypes for anti-B cell cancer agents.

Modeling DNA damage-induced pneumopathy in mice: insight from danger signaling cascades

Radiation-induced pneumonitis and fibrosis represent severe and dose-limiting side effects in the radiotherapy of thorax-associated neoplasms leading to decreased quality of life or - as a consequence of treatment with suboptimal radiation doses - to fatal outcomes by local recurrence or metastatic disease. It is assumed that the initial radiation-induced damage to the resident cells triggers a multifaceted damage-signalling cascade in irradiated normal tissues including a multifactorial secretory program. The resulting pro-inflammatory and pro-angiogenic microenvironment triggers a cascade of events that can lead within weeks to a pronounced lung inflammation (pneumonitis) or after months to excessive deposition of extracellular matrix molecules and tissue scarring (pulmonary fibrosis).The use of preclinical in vivo models of DNA damage-induced pneumopathy in genetically modified mice has helped to substantially advance our understanding of molecular mechanisms and signalling molecules that participate in the pathogenesis of radiation-induced adverse late effects in the lung. Herein, murine models of whole thorax irradiation or hemithorax irradiation nicely reproduce the pathogenesis of the human disease with respect to the time course and the clinical symptoms. Alternatively, treatment with the radiomimetic DNA damaging chemotherapeutic drug Bleomycin (BLM) has frequently been used as a surrogate model of radiation-induced lung disease. The advantage of the BLM model is that the symptoms of pneumonitis and fibrosis develop within 1 month.Here we summarize and discuss published data about the role of danger signalling in the response of the lung tissue to DNA damage and its cross-talk with the innate and adaptive immune systems obtained in preclinical studies using immune-deficient inbred mouse strains and genetically modified mice. Interestingly we observed differences in the role of molecules involved in damage sensing (TOLL-like receptors), damage signalling (MyD88) and immune regulation (cytokines, CD73, lymphocytes) for the pathogenesis and progression of DNA damage-induced pneumopathy between the models of pneumopathy induced by whole thorax irradiation or treatment with the radiomimetic drug BLM. These findings underline the importance to pursue studies in the radiation model(s) if we are to unravel the mechanisms driving radiation-induced adverse late effects.A better understanding of the cross-talk of danger perception and signalling with immune activation and repair mechanisms may allow a modulation of these processes to prevent or treat radiation-induced adverse effects. Vice-versa an improved knowledge of the normal tissue response to injury is also particularly important in view of the increasing interest in combining radiotherapy with immune checkpoint blockade or immunotherapies to avoid exacerbation of radiation-induced normal tissue toxicity.

DNA damage inhibits lateral root formation by up-regulating cytokinin biosynthesis genes in Arabidopsis thaliana

Lateral roots (LRs) are an important organ for water and nutrient uptake from soil. Thus, control of LR formation is crucial in the adaptation of plant growth to environmental conditions. However, the underlying mechanism controlling LR formation in response to external factors has remained largely unknown. Here, we found that LR formation was inhibited by DNA damage. Treatment with zeocin, which causes DNA double-strand breaks, up-regulated several DNA repair genes in the LR primordium (LRP) through the signaling pathway mediated by the transcription factor SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1). Cell division was severely inhibited in the LRP of zeocin-treated sog1-1 mutant, which in turn inhibited LR formation. This result suggests that SOG1-mediated maintenance of genome integrity is crucial for proper cell division during LRP development. Furthermore, zeocin induced several cytokinin biosynthesis genes in a SOG1-dependent manner, thereby activating cytokinin signaling in the LRP. LR formation was less inhibited by zeocin in mutants defective in cytokinin biosynthesis or signaling, suggesting that elevated cytokinin signaling is crucial for the inhibition of LR formation in response to DNA damage. We conclude that SOG1 regulates DNA repair and cytokinin signaling separately and plays a key role in controlling LR formation under genotoxic stress.

Common and distinct mechanisms of induced pulmonary fibrosis by particulate and soluble chemical fibrogenic agents

Pulmonary fibrosis results from the excessive deposition of collagen fibers and scarring in the lungs with or without an identifiable cause. The mechanism(s) underlying lung fibrosis development is poorly understood, and effective treatment is lacking. Here we compared mouse lung fibrosis induced by pulmonary exposure to prototypical particulate (crystalline silica) or soluble chemical (bleomycin or paraquat) fibrogenic agents to identify the underlying mechanisms. Young male C57BL/6J mice were given silica (2 mg), bleomycin (0.07 mg), or paraquat (0.02 mg) by pharyngeal aspiration. All treatments induced significant inflammatory infiltration and collagen deposition, manifesting fibrotic foci in silica-exposed lungs or diffuse fibrosis in bleomycin or paraquat-exposed lungs on day 7 post-exposure, at which time the lesions reached their peaks and represented a junction of transition from an acute response to chronic fibrosis. Lung genome-wide gene expression was analyzed, and differential gene expression was confirmed by quantitative RT-PCR, immunohistochemistry, and immunoblotting for representative genes to demonstrate their induced expression and localization in fibrotic lungs. Canonical signaling pathways, gene ontology, and upstream transcription networks modified by each agent were identified. In particular, these inducers elicited marked proliferative responses; at the same time, silica preferentially activated innate immune functions and the defense against foreign bodies, whereas bleomycin and paraquat boosted responses related to cell adhesion, platelet activation, extracellular matrix remodeling, and wound healing. This study identified, for the first time, the shared and unique genes, signaling pathways, and biological functions regulated by particulate and soluble chemical fibrogenic agents during lung fibrosis, providing insights into the mechanisms underlying human lung fibrotic diseases.

The pathogenesis of bleomycin-induced lung injury in animals and its applicability to human idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a devastating disease of unknown etiology, for which there is no curative pharmacological therapy. Bleomycin, an anti-neoplastic agent that causes lung fibrosis in human patients has been used extensively in rodent models to mimic IPF. In this review, we compare the pathogenesis and histological features of human IPF and bleomycin-induced pulmonary fibrosis (BPF) induced in rodents by intratracheal delivery. We discuss the current understanding of IPF and BPF disease development, from the contribution of alveolar epithelial cells and inflammation to the role of fibroblasts and cytokines, and draw conclusions about what we have learned from the intratracheal bleomycin model of lung fibrosis.

DNA cleavage by oxymyoglobin and cysteine-introduced metmyoglobin

DNA was cleaved oxidatively by oxygenated myoglobin, whereas Lys96Cys metmyoglobin functioned as an artificial nuclease under air by formation of an oxygenated species.

Mechanisms for Structural Variation in the Human Genome

It has been known for several decades that genetic variation involving changes to chromosomal structure (i.e., structural variants) can contribute to disease; however this relationship has been brought into acute focus in recent years largely based on innovative new genomics approaches and technology. Structural variants (SVs) arise from improperly repaired DNA double-strand breaks (DSB). DSBs are a frequent occurrence in all cells and two major pathways are involved in their repair: homologous recombination and non-homologous end joining. Errors during these repair mechanisms can result in SVs that involve losses, gains and rearrangements ranging from a few nucleotides to entire chromosomal arms. Factors such as rearrangements, hotspots and induced DSBs are implicated in the formation of SVs. While de novo SVs are often associated with disease, some SVs are conserved within human subpopulations and may have had a meaningful influence on primate evolution. As the ability to sequence the whole human genome rapidly evolves, the diversity of SVs is illuminated, including very complex rearrangements involving multiple DSBs in a process recently designated as "chromothripsis". Elucidating mechanisms involved in the etiology of SVs informs disease pathogenesis as well as the dynamic function associated with the biology and evolution of human genomes.

The effects of gamma irradiation on growth and expression of genes encoding DNA repair-related proteins in Lombardy poplar (Populus nigra var. italica)

In this study, to elucidate the mechanisms of adaptation and tolerance to ionizing radiation in woody plants, we investigated the various biological effects of γ-rays on the Lombardy poplar (Populus nigra L. var. italica Du Roi). We detected abnormal leaf shape and color, fusion, distorted venation, shortened internode, fasciation and increased axillary shoots in γ-irradiated poplar plants. Acute γ-irradiation with a dose of 100Gy greatly reduced the height, stem diameter and biomass of poplar plantlets. After receiving doses of 200 and 300Gy, all the plantlets stopped growing, and then most of them withered after 4-10 weeks of γ-irradiation. Comet assays showed that nuclear DNA in suspension-cultured poplar cells had been damaged by γ-rays. To determine whether DNA repair-related proteins are involved in the response to γ-rays in Lombardy poplars, we cloned the PnRAD51, PnLIG4, PnKU70, PnXRCC4, PnPCNA and PnOGG1 cDNAs and investigated their mRNA expression. The PnRAD51, PnLIG4, PnKU70, PnXRCC4 and PnPCNA mRNAs were increased by γ-rays, but the PnOGG1 mRNA was decreased. Moreover, the expression of PnLIG4, PnKU70 and PnRAD51 was also up-regulated by Zeocin known as a DNA cleavage agent. These observations suggest that the morphogenesis, growth and protective gene expression in Lombardy poplars are severely affected by the DNA damage and unknown cellular events caused by γ-irradiation.

Telomerase deficiency affects the formation of chromosomal translocations by homologous recombination in Saccharomyces cerevisiae

Telomerase is a ribonucleoprotein complex required for the replication and protection of telomeric DNA in eukaryotes. Cells lacking telomerase undergo a progressive loss of telomeric DNA that results in loss of viability and a concomitant increase in genome instability. We have used budding yeast to investigate the relationship between telomerase deficiency and the generation of chromosomal translocations, a common characteristic of cancer cells. Telomerase deficiency increased the rate of formation of spontaneous translocations by homologous recombination involving telomere proximal sequences during crisis. However, telomerase deficiency also decreased the frequency of translocation formation following multiple HO-endonuclease catalyzed DNA double-strand breaks at telomere proximal or distal sequences before, during and after crisis. This decrease correlated with a sequestration of the central homologous recombination factor, Rad52, to telomeres determined by chromatin immuno-precipitation. This suggests that telomerase deficiency results in the sequestration of Rad52 to telomeres, limiting the capacity of the cell to repair double-strand breaks throughout the genome. Increased spontaneous translocation formation in telomerase-deficient yeast cells undergoing crisis is consistent with the increased incidence of cancer in elderly humans, as the majority of our cells lack telomerase. Decreased translocation formation by recombinational repair of double-strand breaks in telomerase-deficient yeast suggests that the reemergence of telomerase expression observed in many human tumors may further stimulate genome rearrangement. Thus, telomerase may exert a substantial effect on global genome stability, which may bear significantly on the appearance and progression of cancer in humans.

Cellular processing pathways contribute to the activation of etoposide-induced DNA damage responses

Cytotoxic action (tumor cell killing) and carcinogenic side effect (therapy-related secondary leukemia) of etoposide are closely related to its ability in stabilizing topoisomerase II cleavable complex (TOP2cc), a unique form of protein-linked DNA break. How cells process and detect TOP2-concealed DNA damage for the activation of downstream cellular responses remains unclear. Here, we showed proteasomal degradation of both TOP2 isozymes in a transcription-dependent manner upon etoposide treatment. Downregulation of TOP2 was preferentially associated with proteasomal removal of TOP2 in TOP2cc rather than proteolysis of free TOP2. Interestingly, blockage of TOP2 downregulation in TOP2cc also caused reduction in etoposide-induced activation of DNA damage molecules, an observation suggesting that the processing pathways of TOP2cc are involved in activation of etoposide-induced cellular responses. In this regard, we observed two TOP2cc processing pathways, replication- and transcription-initiated processing (RIP and TIP) with proteasome involved in the latter. Importantly, two processing pathways contributed to differential activation of various DNA damage signaling and downstream cellular responses. Etoposide-induced phosphorylation of p53 relied mainly on RIP, whereas activation of Chk1, Chk2 depended largely on TIP. Both RIP and TIP played roles in activating non-homologous end joining pathway, while only RIP modulated etoposide-induced cell killing in a p53-dependent manner. Collectively, our results are consistent with the notion that protein-linked DNA breakage (e.g., TOP2cc) requires processing pathways for initiating downstream DNA damage detection, repair as well as cell death programs.

DNA damage differentially activates regional chromosomal loci for Tn7 transposition in Escherichia coli

The bacterial transposon Tn7 recognizes replicating DNA as a target with a preference for the region where DNA replication terminates in the Escherichia coli chromosome. It was previously shown that DNA double-strand breaks in the chromosome stimulate Tn7 transposition where transposition events occur broadly around the point of the DNA break. We show that individual DNA breaks actually activate a series of small regional hotspots in the chromosome for Tn7 insertion. These hotspots are fixed and become active only when a DNA break occurs in the same region of the chromosome. We find that the distribution of insertions around the break is not explained by the exonuclease activity of RecBCD moving the position of the DNA break, and stimulation of Tn7 transposition is not dependent on RecBCD. We show that other forms of DNA damage, like exposure to UV light, mitomycin C, or phleomycin, also stimulate Tn7 transposition. However, inducing the SOS response does not stimulate transposition. Tn7 transposition is not dependent on any known specific pathway of replication fork reactivation as a means of recognizing DNA break repair. Our results are consistent with the idea that Tn7 recognizes DNA replication involved in DNA repair and reveals discrete regions of the chromosome that are differentially activated as transposition targets.

Mechanistic studies on bleomycin-mediated DNA damage: multiple binding modes can result in double-stranded DNA cleavage

The bleomycins (BLMs) are a family of natural glycopeptides used clinically as antitumor agents. In the presence of required cofactors (Fe(2+) and O(2)), BLM causes both single-stranded (ss) and double-stranded (ds) DNA damage with the latter thought to be the major source of cytotoxicity. Previous biochemical and structural studies have demonstrated that BLM can mediate ss cleavage through multiple binding modes. However, our studies have suggested that ds cleavage occurs by partial intercalation of BLM's bithiazole tail 3' to the first cleavage site that facilitates its re-activation and re-organization to the second strand without dissociation from the DNA where the second cleavage event occurs. To test this model, a BLM A5 analog (CD-BLM) with beta-cyclodextrin attached to its terminal amine was synthesized. This attachment presumably precludes binding via intercalation. Cleavage studies measuring ss:ds ratios by two independent methods were carried out. Studies using [(32)P]-hairpin technology harboring a single ds cleavage site reveal a ss:ds ratio of 6.7 +/- 1.2:1 for CD-BLM and 3.4:1 and 3.1 +/- 0.3:1 for BLM A2 and A5, respectively. In contrast with BLM A5 and A2, however, CD-BLM mediates ds-DNA cleavage through cooperative binding of a second CD-BLM molecule to effect cleavage on the second strand. Studies using the supercoiled plasmid relaxation assay revealed a ss:ds ratio of 2.8:1 for CD-BLM in comparison with 7.3:1 and 5.8:1, for BLM A2 and A5, respectively. This result in conjunction with the hairpin results suggest that multiple binding modes of a single BLM can lead to ds-DNA cleavage and that ds cleavage can occur using one or two BLM molecules. The significance of the current study to understanding BLM's action in vivo is discussed.

Assembly of nuclear matrix-bound protein complexes involved in non-homologous end joining is induced by inhibition of DNA topoisomerase II

Topoisomerases maintain the DNA structure by relieving the torsional stress and alleviating other topological problems occurring in DNA during transcription and replication. Topoisomerase II appears to have a close association with the family of proteins involved in the organization of chromatin in a series of loops on the proteinaceous chromosomal matrix. Beyond its physiological functions, topoisomerase II is the target for some of the most active anticancer drugs. Inhibition of the topoisomerase II function can result in DNA double-strand breaks (DSBs) and, thus, lead to chromosomal translocations. The earliest event during DSB repair is phosphorylation of histone H2AX at S139 (so-called gammaH2AX) which is believed to serve as a focal point for the assembly of repair proteins at the DSB. In this work, we have demonstrated the formation of gammaH2AX foci in two human cell lines--K562 and HeLa--after suppression of topoisomerase II activity with etoposide. Furthermore, these foci remained visible at nuclear matrices and colocalized with the major components of non-homologous end joining (NHEJ) system of DSBs repair. Thus, inhibition of topoisomerase II activity triggers assembly of NHEJ complexes at the nuclear matrix.

Bleomycins: towards better therapeutics

Bleomycins are a family of glycopeptide antibiotics that have potent antitumour activity against a range of lymphomas, head and neck cancers and germ-cell tumours. The therapeutic efficacy of the bleomycins is limited by development of lung fibrosis. The cytotoxic and mutagenic effects of the bleomycins are thought to be related to their ability to mediate both single-stranded and double-stranded DNA damage, which requires the presence of specific cofactors (a transition metal, oxygen and a one-electron reductant). Progress in understanding the mechanisms involved in the therapeutic efficacy of the bleomycins and the unwanted toxicity and elucidation of the biosynthetic pathway of the bleomycins sets the stage for developing a more potent, less toxic therapeutic agent.

Solution structure of the hydroperoxide of Co(III) phleomycin complexed with d(CCAGGCCTGG)2: evidence for binding by partial intercalation

The bleomycins (BLMs) are natural products that in the presence of iron and oxygen bind to and cause single-strand and double-strand cleavage of DNA. The mode(s) of binding of the FeBLMs that leads to sequence-specific cleavage at pyrimidines 3' to guanines and chemical-specific cleavage at the C-4' H of the deoxyribose of the pyrimidine has remained controversial. 2D NMR studies using the hydroperoxide of CoBLM (HOO-CoBLM) have demonstrated that its bithiazole tail binds by partial intercalation to duplex DNA. Studies with ZnBLM demonstrate that the bithiazole tail binds in the minor groove. Phleomycins (PLMs) are BLM analogs in which the penultimate thiazolium ring of the bithiazole tail is reduced. The disruption of planarity of this ring and the similarities between FePLM- and FeBLM-mediated DNA cleavage have led Hecht and co-workers to conclude that a partial intercalative mode of binding is not feasible. The interaction of HOO-CoPLM with d(CCAGGCCTGG)2 has therefore been investigated. Binding studies indicate a single site with a K(d) of 16 micro M, 100-fold greater than HOO-CoBLM for the same site. 2D NMR methods and molecular modeling using NMR-derived restraints have led to a structural model of HOO-CoPLM complexed to d(CCAGGCCTGG)2. The model reveals a partial intercalative mode of binding and the basis for sequence specificity of binding and chemical specificity of cleavage. The importance of the bithiazoles and the partial intercalative mode of binding in the double-strand cleavage of DNA is discussed.

Total synthesis of deamido bleomycin a(2), the major catabolite of the antitumor agent bleomycin

Metabolic inactivation of the antitumor antibiotic bleomycin is believed to be mediated exclusively via the action of bleomycin hydrolase, a cysteine proteinase that is widely distributed in nature. While the spectrum of antitumor activity exhibited by the bleomycins is believed to reflect the anatomical distribution of bleomycin hydrolase within the host, little has been done to characterize the product of the putative inactivation at a chemical or biochemical level. The present report describes the synthesis of deamidobleomycin demethyl A(2) (3) and deamido bleomycin A(2) (4), as well as the respective aglycones. These compounds were all accessible via the key intermediate N(alpha)-Boc-N(beta)-[1-amino-3(S)-(4-amino-6-carboxy-5-methylpyrimidin-2-yl)propion-3-yl]-(S)-beta-aminoalanine tert-butyl ester (16). Synthetic deamido bleomycin A(2) was shown to be identical to the product formed by treatment of bleomycin A(2) with human bleomycin hydrolase, as judged by reversed-phase HPLC analysis and (1)H NMR spectroscopy. Deamido bleomycin A(2) was found to retain significant DNA cleavage activity in DNA plasmid relaxation assays and had the same sequence selectivity of DNA cleavage as bleomycin A(2). The most significant alteration of function noted in this study was a reduction in the ability of deamido bleomycin A(2) to mediate double-strand DNA cleavage, relative to that produced by BLM A(2).

Permanent cell cycle arrest in asynchronously proliferating normal human fibroblasts treated with doxorubicin or etoposide but not camptothecin

Damage to DNA has been implicated in the induction of permanent cell cycle arrest or premature senescence in normal human fibroblasts. We tested the ability of a group of cancer chemotherapeutic agents or related compounds, which can cause DNA double-strand breaks (DSBs) directly or indirectly, to induce a permanent cell cycle arrest in normal proliferating fibroblasts. A brief treatment with etoposide, doxorubicin, cisplatin, or phleomycin D1 induced a block to S phase entry sustained through 15 days. Lower levels of these drugs did not induce appreciable levels of transient cell cycle arrest. Higher concentrations caused cell death that lacked the DNA degradation characteristic of apoptosis. Camptothecin, an agent that causes DNA single-strand breaks, which are converted to DSBs during S phase, was able to induce an efficient, but only transient, cell cycle arrest in these normal cells. The cells did not enter S phase until after removal of the camptothecin. These findings support the idea that permanent cell cycle arrest and cell death are typical reactions of these normal cells to drugs that can cause DSBs. In addition, we report data consistent with the concept that both actinomycin D and doxorubicin are sequestered by cells and slowly released in active form. This is consistent with the observation that both these drugs bind reversibly to intracellular components.

A Saccharomyces cerevisiae phleomycin-sensitive mutant, ph140, is defective in the RAD6 DNA repair gene

The antibiotic bleomycin is used as an anticancer agent for treating a variety of tumours. The antitumour effect of bleomycin is related to its ability to produce lesions such as apurinic/apyrimidinic sites and single- and double-strand breaks in the cellular DNA. Phleomycin is a structurally related form of bleomycin, but it is not used as an anticancer agent. While phleomycin can also damage DNA, neither the exact nature of these DNA lesions nor the cellular process that repairs phleomycin-induced DNA lesions is known. As a first step to understand how eukaryotic cells provide resistance to phleomycin, we used the yeast Saccharomyces cerevisiae as a model system. Several phleomycin-sensitive mutants were generated following gamma-radiation treatment and among these mutants, ph140 was found to be the most sensitive to phleomycin. Molecular analysis revealed that the mutant ph140 harbored a mutation in the DNA repair gene RAD6. Moreover, a functional copy of the RAD6 gene restored full phleomycin resistance to strain ph140. Our findings indicate that the RAD6 protein is essential for yeast cellular resistance to phleomycin.

Synthesis and evaluation of potential N pi and N sigma metal chelation sites within the beta-hydroxy-L-histidine subunit of bleomycin A2: functional characterization of imidazole N pi metal complexation

The synthesis and evaluation of 4 and 5, fully functionalized deglycobleomycin A2 (2) analogues incorporating an oxazole and a pyrrole in place of the beta-hydroxy-L-histidine imidazole, are detailed. The oxazole agent is only capable of Npi metal complexation through a form related to the N1-H imidazole tautomer of bleomycin A2 (1) while the pyrrole agent may potentially mimic the Nsigma metal complexation capabilities of the imidazole N3-H tautomer. Metal complexes (Fe-II, Fe-III) of 4 and 5 were found to cleave duplex DNA in the presence of O2 (Fe-II) or H2O2 (Fe-III). The oxazole agent 4 which is incapable of Nsigma metal chelation was found to behave analogous to, albeit slightly less effectively than, deglycobleomycin A2 resulting in the characteristic 5'-GC/5'-GT sequence selective cleavage of duplex DNA directly confirming that imidazole/oxazole Npi metal chelation is sufficient for functional reactivity. Importantly, the effective substitution of the oxazole O-1 for the histidine N-1 further illustrates that this group does not require deprotonation upon metal complexation, oxygen activation, or the ensuing oxidation reactions, that the functional bleomycin A2 tautomer is the imidazole N'-H tautomer, and that the imidazole N'-H functionality is not contributing to the polynucleotide recognition through H-bonding to the phosphate backbone or nucleotide bases. In contrast, the pyrrole agent 5 which is incapable of Npi metal chelation, but possesses the capabilities of functioning as a Nsigma metal donor was also found to cleave duplex DNA, but does so in a nonsequence selective fashion with a significantly reduced efficiency and a diminished double to single strand cleavage ratio both only slightly above that of background iron itself. These observations are analogous to those made with 3 which lacks the imidazole altogether and further support the observations that Nsigma coordination, not Npi coordination, of the imidazole is required for the functional activity of bleomycin A2.

Genetic changes and bioassays in bleomycin- and phleomycin-treated cells, and their relationship to chromosomal breaks

The recombinogenicity of damaged chromosomes in diploid Saccharomyces cerevisiae cells treated with bleomycin and structurally related phleomycin was measured, along with aneuploidy and mutation events. Phleomycin was substantially (up to 26-fold) more effective than bleomycin in producing genetic changes at all concentrations, even when colony-forming abilities of cells growing in the presence of bleomycin or phleomycin were similar. These results suggest that the DNA lesions produced by the two structurally related analogs could differ in their nature or frequency, or could be processed differently by the cells. Bioassays were developed and used to compare the cytotoxicities of freshly dissolved bleomycin and phleomycin with the cytotoxicities of lysates prepared from bleomycin- and phleomycin-treated cells. Unexpectedly, lysates prepared from bleomycin-treated cells were 1.5-3.5 times more cytotoxic than freshly dissolved bleomycin after 45-min treatments (3-33 x 10(-6) M). In contrast, lysates prepared from phleomycin-treated cells were 3-38 times less cytotoxic than freshly dissolved phleomycin (0.5-6.4 x 10(-6) M). Cytotoxicities of all lysates were higher after 36-h treatments than after 45-min treatments. At 3.3 x 10(-6) M, this increase was eightfold for bleomycin and 15-fold for phleomycin. Nevertheless, lysates from phleomycin-treated cells were considerably more cytotoxic than lysates from bleomycin-treated cells or freshly prepared bleomycin, consistent with the higher effectiveness of phleomycin than bleomycin in producing chromosomal breaks, genetic changes, and cell killing.

Hypoxia and drug resistance

Biologically and therapeutically important hypoxia occurs in many solid tumor masses. Hypoxia can be a direct cause of therapeutic resistance because some drugs and radiation require oxygen to be maximally cytotoxic. Cellular metabolism is altered under hypoxic conditions. Hypoxia can result in drug resistance indirectly if under this condition cells more effectively detoxify the drug molecules. Finally, there is evidence that hypoxia can enhance genetic instability in tumor cells thus allowing more rapid development of drug resistance cells. The current review describes the effects of hypoxia on tumor response to a variety of anti-cancer agents and also describes progress toward therapeutically useful methods of delivering oxygen to tumors in an effort to overcome therapeutic resistance due to hypoxia. Finally, the use of hypoxic cell selective cytotoxic agents as a means of addressing hypoxic 'drug resistance' is discussed.

Damage to lymphocytes by X-ray and bleomycin measured with the cytokinesis-block micronucleus technique

Chromosome damage induced by X-irradiation or bleomycin was measured using the cytokinesis-block micronucleus assay in the peripheral blood lymphocytes of 6 newborn, 8 young and 10 elderly individuals. An increase in the frequency of spontaneous micronuclei with age was observed. There was no difference in the X-irradiation-induced micronucleus frequency between the 3 groups. There was a significant increase with age in the number of micronuclei induced by bleomycin. Kinetochore-labelling studies revealed that the percentage of kinetochore-positive induced micronuclei was higher for bleomycin (36.2-43.3%) than for X-irradiation (17.1-19.7%). The age-related increase in frequency of spontaneous or bleomycin-induced micronuclei was due to increases in both kinetochore-positive and kinetochore-negative micronuclei. The frequency of kinetochore-positive or -negative micronuclei induced by X-irradiation was not different between the 3 age groups. These results suggest that bleomycin is more potent in inducing whole-chromosome loss than X-rays, and that lymphocytes from aged individuals are more sensitive to bleomycin in terms of both chromosome breakage and whole chromosome loss.

Defective repair of DNA single- and double-strand breaks in the bleomycin- and X-ray-sensitive Chinese hamster ovary cell mutant, BLM-2

Using filter elution techniques, we have measured the level of induced single- and double-strand DNA breaks and the rate of strand break rejoining following exposure of two Chinese hamster ovary (CHO) cell mutants to bleomycin or neocarzinostatin. These mutants, designated BLM-1 and BLM-2, were isolated on the basis of hypersensitivity to bleomycin and are cross-sensitive to a range of other free radical-generating agents, but exhibit enhanced resistance to neocarzinostatin. A 1-h exposure to equimolar doses of bleomycin induces a similar level of DNA strand breaks in parental CHO-K1 and mutant BLM-1 cells, but a consistently higher level is accumulated by BLM-2 cells. The rate of rejoining of bleomycin-induced single- and double-strand DNA breaks is slower in BLM-2 cells than in CHO-K1 cells. BLM-1 cells show normal strand break repair kinetics. The level of single- and double-strand breaks induced by neocarzinostatin is lower in both BLM-1 and BLM-2 cells than in CHO-K1 cells. The rate of repair of neocarzinostatin-induced strand breaks is normal in BLM-1 cells but retarded somewhat in BLM-2 cells. Thus, there is a correlation between the level of drug-induced DNA damage in BLM-2 cells and the bleomycin-sensitive, neocarzinostatin resistant phenotype of this mutant. Strand breaks induced by both of these agents are also repaired with reduced efficiency by BLM-2 cells. The neocarzinostatin resistance of BLM-1 cells appears to be a consequence of a reduced accumulation of DNA damage. However, the bleomycin-sensitive phenotype of BLM-1 cells does not apparently correlate with any alteration in DNA strand break induction or repair, as analysed by filter elution techniques, suggesting an alternative mechanism of cell killing.

Substrate specificity of bleomycin hydrolase

Bleomycin (BLM) hydrolase is believed to protect both malignant and normal tissue from the toxicity of the antitumor drug BLM. Little is known about the substrate specificity of BLM hydrolase. Thus, we developed ion-paired reverse phase high speed liquid chromatography systems to assay for the metabolism of several BLM analogs. We found that BLM A2, BLM B2, tallysomycin S10b (TLM S10b), peplomycin (PEP), butylamino-3-propylamino-3-propylamine bleomycin (BAPP), deglyco bleomycin A2 (dgBLM A2) and bleomycinic acid were each metabolized by rabbit lung BLM hydrolase to a single metabolite. When compared to their corresponding parent compounds, these metabolites were 6- to 35-fold less potent in their ability to inhibit the proliferation of A-253 human head and neck squamous carcinoma cells in culture. Furthermore, we found that substitutions in various regions of the BLM molecule greatly affected the kinetic parameters of BLM hydrolase. For example, the Km with BLM B2 (0.056 +/- 0.005 mM) was 15-fold lower than that seen with BLM A2 (0.83 +/- 0.11 mM). In contrast, the Vmax was not affected markedly by these terminal amine substitutions but was influenced greatly by deletion of the carbohydrate groups of BLM. For example, a 4-fold higher Vmax was observed with dgBLM A2 compared to BLM A2. Thus, these results demonstrate that BLM hydrolase can recognize and metabolize a broad spectrum of BLM analogs regardless of their structural features. This enzymatic conversion resulted in the inactivation of the BLMs as demonstrated by a substantial decrease in their cytotoxicity. Furthermore, the terminal amine and carbohydrate regions, respectively, dictate the apparent affinity and the rate of metabolism of BLM hydrolase substrates.

Bleomycin and X-ray-hypersensitive Chinese hamster ovary cell mutants: genetic analysis and cross-resistance to neocarzinostatin

We have previously reported the isolation of 3 mutants of Chinese hamster ovary cells which exhibit hypersensitivity to bleomycin. 2 mutants were isolated on the basis of bleomycin-sensitivity [designated BLM-1 and BLM-2, Robson et al., Cancer Res., 45 (1985) 5304-5309] and 1 as adriamycin-sensitive [ADR-1, Robson et al., Cancer Res., 47 (1987) 1560-1565]. Because bleomycin generates DNA-strand breaks via a free-radical mechanism, we have studied the survival response of these mutants to a range of drugs which also generate free radicals and consequently DNA-strand breaks. The mutants are all hypersensitive to phleomycin, which differs from bleomycin in being unable to intercalate due to a modified bithiazole moiety. However, BLM-2 cells alone are hypersensitive to pepleomycin, a semi-synthetic bleomycin analogue. In contrast, BLM-1 cells are more sensitive than BLM-2 to streptonigrin (which operates via a hydroquinone intermediate). ADR-1 cells show wild-type resistance to streptonigrin. The results obtained with neocarzinostatin, an antibiotic requiring thiol activation, are unusual in that both BLM-1 and BLM-2 are approximately 3-fold more resistant than parental cells. However, the steady-state intracellular level of the major non-protein thiol, glutathione, is not altered in BLM-1 or BLM-2 cells. ADR-1 cells show essentially wild-type resistance to neocarzinostatin. Analysis of cell hybrids shows that BLM-1 and BLM-2 cells are phenotypically recessive in combination with parental CHO-K1 cells and represent different genetic complementation groups not only from one another, but also from the bleomycin-sensitive mutant xrs-6, isolated on the basis of X-ray sensitivity by Jeggo and Kemp [Mutation Res., 112 (1983) 313-319]. These results indicate that at least 3 gene products are involved in cellular protection against bleomycin toxicity in mammalian cells.

Breakage of double-stranded DNA due to single-stranded nicking

Enzymes such as pancreatic deoxyribonuclease (DNase I) nick the single strands of double-stranded DNA. Two nicks sufficiently close on opposite strands will lead to breakage of the DNA molecule. This paper gives a mathematical model for the breakage of circular, supercoiled DNA under the action of an enzyme which nicks at random sites (or at preferred sites, these being in abundance and randomly positioned around the circle). After the first nick the DNA loses its supercoiled structure; after many nicks it breaks to become topologically linear; further nicks lead to fragmentation of this linear form. Formulae are given for the proportions of DNA molecules in each of the four classes: supercoiled; nicked but still circular; linear; fragmented. Formulae are also presented for the case when there is, in addition to nicking, simultaneous action of an endonuclease which produces direct double-stranded breaks in the DNA. Finally, a general theory is given for the case where a third type of enzyme, topoisomerase I, is operative, with all three DNA modifications taking place simultaneously.

Biological and biochemical activities of the novel antitumor antibiotic PD 114,759 and related derivatives

A complex of novel and exceptionally potent antibiotics has been evaluated for antitumor activity in vitro and in vivo and characterized with regard to their ability to cause DNA strand scission. The major component, PD 114,759, was quite active against all in vitro tumor systems including the human tumors, MCF-7 breast, HCT-8 colon, and A549 lung and the murine tumors M16/c mammary, Lewis lung, Pan 02 pancreas and L1210 leukemia. ID50 values ranged from 2-57 pg/ml. In vivo this agent produced significant increases of host life spans in mice bearing L1210 leukemia, B16 melanoma and the M5076 sarcoma. Further, it inhibited growth of subcutaneous implants of the Ridgway osteogenic sarcoma by 80% and growth of the MX-1 human mammary xenograft by 90-95%. PD 114,759, however, had no activity against the colon adenocarcinoma 11a or mammary adenocarcinoma 16c. Chinese hamster ovary cells exposed for 24 hours to concentrations of PD 114,759 ranging from 18 to 37 pg/ml accumulated in the S and G2+M phases of the cell cycle with a corresponding decrease in G1. Higher concentrations of drug apparently stopped any progression through the cell cycle. PD 114,759 caused significant DNA single strand breaks in L1210 cells exposed for 1 hour to drug concentrations as low as 20 pg/ml and the frequency of these lesions increased in proportion to the drug concentration. A portion of these DNA breaks appeared to be associated with protein. In contrast, no double strand DNA breaks were detected at the highest drug concentration tested (100 pg/ml).

Quantification of circular DNA breakage by fluorescence scanning of ethidium bromide-stained horizontal gels: application to the effect of intercalating agents

Direct scanning of the fluorescence of DNA in ethidium bromide-stained agarose gels allowed the quantification of closed and open circular DNA forms. Fluorescence of form I was higher than expected compared to form II. Application of this technique is shown for an intercalating drug treatment of DNA.

Isolation of cell cycle-dependent gamma ray-sensitive Chinese hamster ovary cell

A technique for the isolation of gamma ray-sensitive Chinese hamster ovary (CHO) cell mutants is described, which uses nylon cloth replica plating and photography with dark-field illumination to directly monitor colonies for growth after gamma irradiation. Two gamma ray-sensitive mutants were isolated using this method. One of these cells (XR-1) had a two-slope survival curve: an initial steep slope and then a flattening of the curve at about 10% survival. Subsequently, it was found that this cell is sensitive to gamma irradiation in G1, early S, and late G2 phases of the cell cycle, whereas in the resistant phase (late S phase) its survival approaches that of the parental cells. The D37 in the sensitive G1 period is approximately 30 rads, compared with 300 rads of the parental cell. This mutant cell is also sensitive to killing by the DNA breaking agent, bleomycin, but is relatively insensitive to UV light and ethyl methane sulfonate, suggesting that the defect is specific for agents that produce DNA strand breakage.

Use of a rapid DNA sequencing system to demonstrate the induction of frameshift mutations by bleomycin

The rapid DNA sequencing system based on the single-stranded bacteriophage M13 and the chain-terminator method has been used to look directly for mutational alterations. A small DNA fragment that primes DNA synthesis through the N-terminal 200 base pairs of the beta-galactosidase gene was prepared, and used to detect changes in base sequence among phages that give white plaques after treatment of the host cells with bleomycin. Bleomycin treatment of E. coli in which M13 mp2 was growing gave an increase in white plaque frequency. DNA sequence analysis of phage from 7 independent mutant plaques showed them all to have a frameshift mutation.

Differences in the cleavage of DNA by bleomycin under light (300-350 nm) and dark conditions

Incubation of supercoiled plasmid DNA (both pBR 322 and pPLC-28) with increasing concentrations of bleomycin in the presence and absence of light (300-350 nm) showed extensive conversion of supercoiled to linear DNA which occurred at much lower levels of bleomycin (BLM) in the light than in the dark. Preliminary results indicate that superoxide dismutase (SOD) may offer DNA more protection against bleomycin in the dark than in the light conditions.

Minor role of lipid peroxidation in acute bleomycin toxicity in rats

Bleomycin was injected i.p. in rats, and the amount of expired ethane which indicates lipid peroxidation was followed up for 78 h. Compared to controls neither 1 x 30 mg/kg and 2 x 30 mg/kg nor 1 x 70 mg/kg bleomycin led to increased ethane expiration, although body weight loss indicated toxicity. That pulmonary toxicity had been developed due to the acute bleomycin treatment could be demonstrated by histological examinations of lungs of the animals of the highest dosage group. The combined treatment of rats with bleomycin and ferrous ions neither resulted in an increase of ethane expired compared to that of the ferrous ion-treated animals. Rather a decrease was observed. Our results indicate that acute bleomycin toxicity is not associated with increased lipid peroxidation. Furthermore, our data suggest that the bleomycin-ferrous-complex does not initiate lipid peroxidation in vivo.