Topoisomerase II inhibition by the bioactivated benzene metabolite hydroquinone involves multiple mechanisms

Hydroquinone Ecotoxicity: Unveiling Risks in Soil and River Ecosystems with Insights into Microbial Resilience

Despite widespread industrial use, the environmental safety of hydroquinone (HQ), a benzene compound from plants used in processes like cosmetics, remains uncertain. This study evaluated the ecotoxicological impact of HQ on soil and river environments, utilizing non-target indicator organisms from diverse trophic levels: Daphnia magna, Aliivibrio fischeri, Allium cepa, and Eisenia fetida. For a more environmentally realistic assessment, microbial communities from a river and untreated soil underwent 16S rRNA gene sequencing, with growth and changes in community-level physiological profiling assessed using Biolog EcoPlate™ assays. The water indicator D. magna exhibited the highest sensitivity to HQ (EC50 = 0.142 µg/mL), followed by A. fischeri (EC50 = 1.446 µg/mL), and A. cepa (LC50 = 7.631 µg/mL), while E. fetida showed the highest resistance (EC50 = 234 mg/Kg). Remarkably, microbial communities mitigated HQ impact in both aquatic and terrestrial environments. River microorganisms displayed minimal inhibition, except for a significant reduction in polymer metabolism at the highest concentration (100 µg/mL). Soil communities demonstrated resilience up to 100 µg/mL, beyond which there was a significant decrease in population growth and the capacity to metabolize carbohydrates and polymers. Despite microbial mitigation, HQ remains highly toxic to various trophic levels, emphasizing the necessity for environmental regulations.

Linking Benzene, in Utero Carcinogenicity and Fetal Hematopoietic Stem Cell Niches: A Mechanistic Review

Previous research reported that prolonged benzene exposure during in utero fetal development causes greater fetal abnormalities than in adult-stage exposure. This phenomenon increases the risk for disease development at the fetal stage, particularly carcinogenesis, which is mainly associated with hematological malignancies. Benzene has been reported to potentially act via multiple modes of action that target the hematopoietic stem cell (HSCs) niche, a complex microenvironment in which HSCs and multilineage hematopoietic stem and progenitor cells (HSPCs) reside. Oxidative stress, chromosomal aberration and epigenetic modification are among the known mechanisms mediating benzene-induced genetic and epigenetic modification in fetal stem cells leading to in utero carcinogenesis. Hence, it is crucial to monitor exposure to carcinogenic benzene via environmental, occupational or lifestyle factors among pregnant women. Benzene is a well-known cause of adult leukemia. However, proof of benzene involvement with childhood leukemia remains scarce despite previously reported research linking incidences of hematological disorders and maternal benzene exposure. Furthermore, accumulating evidence has shown that maternal benzene exposure is able to alter the developmental and functional properties of HSPCs, leading to hematological disorders in fetus and children. Since HSPCs are parental blood cells that regulate hematopoiesis during the fetal and adult stages, benzene exposure that targets HSPCs may induce damage to the population and trigger the development of hematological diseases. Therefore, the mechanism of in utero carcinogenicity by benzene in targeting fetal HSPCs is the primary focus of this review.

Multiple air pollutants exposure and leukaemia incidence in Tehran, Iran from 2010 to 2016: a retrospective cohort study

OBJECTIVE

Leukaemia is one of the most common cancers and may be associated with exposure to environmental carcinogens, especially outdoor air pollutants. The objective of this study was to investigate the association of ambient air pollution and leukaemia in Tehran, Iran.

DESIGN

In this retrospective cohort study, data about the residential district of leukaemia cases diagnosed from 2010 to 2016 were inquired from the Ministry of Health cancer database. Data from a previous study were used to determine long-term average exposure to different air pollutants in 22 districts of Tehran. Latent profile analysis (LPA) was used to classify pollutants in two exposure profiles. The association between air pollutants and leukaemia incidence was analysed by negative binomial regression.

SETTING

Twenty-two districts of Tehran megacity.

PARTICIPANTS

Patients with leukaemia.

OUTCOME MEASURES

The outcome variables were incidence rate ratios (IRR) of acute myeloid and lymphoid leukaemia across the districts of Tehran.

RESULTS

The districts with higher concentrations for all pollutants were near the city centre. The IRR was positive but non-significant for most of the air pollutants. However, annual mean NOx was directly and significantly associated with total leukaemia incidence in the fully adjusted model (IRR (95% CI): 1.03 (1.003 to 1.06) per 10 ppb increase). Based on LPA, districts with a higher multiple air-pollutants profile were also associated with higher leukaemia incidence (IRR (95% CI): 1.003 (0.99 to 1.007) per 1 ppb increase).

CONCLUSIONS

Our study shows that districts with higher air pollution (nitrogen oxides and multipollutants) have higher incidence rates of leukaemia in Tehran, Iran. This study warrants conducting further research with individual human data and better control of confounding.

Immunosuppression characterized by increased Treg cell and IL-10 levels in benzene-induced hematopoietic toxicity mouse model

Benzene is a typical hematopoietic toxic substance, that can cause serious blood and circulatory system diseases such as aplastic anemia, myelodysplastic syndrome and acute myeloid leukemia, but the immunological mechanism by which this occurs is not clear. T helper cells play a key role in regulating the immune balance in the body. In this study, benzene-induced hematopoietic toxicity BALB/c mice model was established, and changes in immune organs and T helper cell subsets (Th1, Th2, Th17 and Treg cells) were explored. At 28 days after subcutaneous injection of 150 mg/kg benzene, mice showed pancytopenia and obvious pathological damage to the bone marrow, spleen, and thymus. Flow cytometry revealed that the number of CD4⁺CD25⁺Foxp3⁺ Treg cells in the spleen increased significantly. The level of IL-10 in the spleen, serum, and bone marrow increased, while the levels of IL-17 in the spleen and serum decreased. Furthermore, the levels of CD4 and CD8 proteins in the spleen decreased. Immunofluorescence results showed that levels of Foxp3, a specific transcription factor that induced the differentiation of Treg cells, increased after exposure to benzene. Our results demonstrate that immunosuppression occurred in the benzene-induced hematopoietic toxicity model mice, and Treg cells and secreted IL-10 may play a key role in the process.

Etoposide, an anticancer drug involved in therapy-related secondary leukemia: Enzymes at play

Etoposide is a semi-synthetic glycoside derivative of podophyllotoxin, also known as VP-16. It is a widely used anticancer medicine in clinics. Unfortunately, high doses or long-term etoposide treatment can induce therapy-related leukemia. The mechanism by which etoposide induces secondary hematopoietic malignancies is still unclear. In this article, we review the potential mechanisms of etoposide induced therapy-related leukemia. Etoposide related leukemogenesis is known to depend on reactive oxidative metabolites of etoposide, notably etoposide quinone, which interacts with cellular proteins such as topoisomerases II (TOP2), CREB-binding protein (CREBBP), and T-Cell Protein Tyrosine Phosphatase (TCPTP). CYP3A4 and CYP3A5 metabolize etoposide to etoposide catechol, which readily oxidizes to etoposide quinone. As a poison of TOP2 enzymes, etoposide and its metabolites induce DNA double-stranded breaks (DSB), and the accumulation of DSB triggers cell apoptosis. If the cell survives, the DSB gives rise to the likelihood of faulty DNA repair events. The gene translocation could occur in mixed-lineage leukemia (MLL) gene, which is well-known in leukemogenesis. Recently, studies have revealed that etoposide metabolites, especially etoposide quinone, can covalently bind to cysteines residues of CREBBP and TCPTP enzymes, . This leads to enzyme inhibition and further affects histone acetylation and phosphorylation of the JAK-STAT pathway, thus putatively altering the proliferation and differentiation of hematopoietic stem cells (HSC). In brief, current studies suggest that etoposide and its metabolites contribute to etoposide therapy-related leukemia through TOP2 mediated DSB and impairs specific enzyme activity, such as CREBBP and TCPTP.

Topoisomerase II Poisons: Converting Essential Enzymes into Molecular Scissors

The extensive length, compaction, and interwound nature of DNA, together with its controlled and restricted movement in eukaryotic cells, create a number of topological issues that profoundly affect all of the functions of the genetic material. Topoisomerases are essential enzymes that modulate the topological structure of the double helix, including the regulation of DNA under- and overwinding and the removal of tangles and knots from the genome. Type II topoisomerases alter DNA topology by generating a transient double-stranded break in one DNA segment and allowing another segment to pass through the DNA gate. These enzymes are involved in a number of critical nuclear processes in eukaryotic cells, such as DNA replication, transcription, and recombination, and are required for proper chromosome structure and segregation. However, because type II topoisomerases generate double-stranded breaks in the genetic material, they also are intrinsically dangerous enzymes that have the capacity to fragment the genome. As a result of this dualistic nature, type II topoisomerases are the targets for a number of widely prescribed anticancer drugs. This article will describe the structure and catalytic mechanism of eukaryotic type II topoisomerases and will go on to discuss the actions of topoisomerase II poisons, which are compounds that stabilize DNA breaks generated by the type II enzyme and convert these essential enzymes into "molecular scissors." Topoisomerase II poisons represent a broad range of structural classes and include anticancer drugs, dietary components, and environmental chemicals.

Exposure to PM2.5 constituents and risk of adult leukemia in Denmark: A population-based case-control study

BACKGROUND

Leukemia is one of the most common forms of hematologic malignancy, which can affect people of all ages. We previously showed an association between exposure to ambient particulate matter 2.5 μg (PM_2.5) and risk for leukemia in adults. The aim of this study was to investigate which PM_2.5 constituents were responsible for our previous observation.

METHODS

This is a nationwide register-based case-control study. We identified 14,983 persons diagnosed with leukemia at age 20 or above, 1989-2014, in the Danish Cancer Registry. We selected up to four sex and age-matched controls per case at random from the entire Danish population (n = 51,613). We modelled concentrations of ambient PM_2.5 and its constituents at the addresses of cases and controls for the 10-year period before index date with a state-of-the-art multiscale air pollution modeling system. We used conditional logistic regression to estimate odds ratios (ORs) adjusted for individual and neighborhood level socio-demographic variables.

RESULT

The results showed higher risk for overall leukemia in association with interquartile range exposure to PM_2.5 (OR = 1.09; 95% CI: 1.02, 1.17), black carbon (BC) (OR = 1.02; 95% CI: 1.00, 1.03), secondary inorganic aerosols (SIA) (OR = 1.15; 95% CI: 1.03, 1.29) and its components ammonium (NH₄) (OR = 1.08; 95% CI: 1.00, 1.17) and nitrate (NO₃) (OR = 1.08; 95% CI: 1.02, 1.14). In leukemia subtype analysis, statistically significant associations were found for AML with PM_2.5 (OR = 1.14; 95% CI: 1.00, 1.29), BC (OR = 1.03; 95% CI: 1.00, 1.07), SIA (OR = 1.23; 95% CI: 1.01, 1.51), NH₄ (OR = 1.16; 95% CI: 1.01, 1.34) and NO₃ (OR = 1.12; 95% CI: 1.01, 1.24). The association between PM_2.5 and leukemia persisted in two pollutants models including sum of primary emitted black and organic carbon (BC + OC), secondary organic aerosols (SOA), or sea-salt. The association between black carbon (BC) and leukemia persisted in two pollutants models including organic carbon (OC). The three pollutant model with sulfate (SO₄), NH₄ and NO₃ showed an association with NO₃ but not with SO₄ or NH₄.

CONCLUSION

Ambient concentrations of the PM_2.5 components BC, NH₄ and NO₃ at the residence showed associations with risk of incident leukemia in adults.

1,2-Naphthoquinone as a Poison of Human Type II Topoisomerases

1,2-Naphthoquinone, a secondary metabolite of naphthalene, is an environmental pollutant found in diesel exhaust particles that displays cytotoxic and genotoxic properties. Because many quinones have been shown to act as topoisomerase II poisons, the effects of this compound on DNA cleavage mediated by human topoisomerase IIα and IIβ were examined. The compound increased the levels of double-stranded DNA breaks generated by both enzyme isoforms and did so better than a series of naphthoquinone derivatives. Furthermore, 1,2-naphthoquinone was a more efficacious poison against topoisomerase IIα than IIβ. Topoisomerase II poisons can be classified as interfacial (which interact noncovalently at the enzyme-DNA interface and increase DNA cleavage by blocking ligation) or covalent (which adduct the protein and increase DNA cleavage by closing the N-terminal gate of the enzyme). Therefore, experiments were performed to determine the mechanistic basis for the actions of 1,2-naphthoquinone. In contrast to results with etoposide (an interfacial poison), the activity of 1,2-naphthoquinone against topoisomerase IIα was abrogated in the presence of sulfhydryl and reducing agents. Moreover, the compound inhibited cleavage activity when incubated with the enzyme prior to the addition of DNA and induced virtually no cleavage with the catalytic core of the enzyme. It also induced stable covalent topoisomerase IIα-DNA cleavage complexes and was a partial inhibitor of DNA ligation. Findings were also consistent with 1,2-naphthoquinone acting as a covalent poison of topoisomerase IIβ; however, mechanistic studies with this isoform were less conclusive. Whereas the activity of 1,2-naphthoquinone was blocked in the presence of a sulfhydryl reagent, it was much less sensitive to the presence of a reducing agent. Furthermore, the reduced form of 1,2-naphthoquinone, 1,2-dihydroxynaphthalene, displayed high activity against the β isoform. Taken together, results suggest that 1,2-naphthoquinone increases topoisomerase II-mediated double-stranded DNA scission (at least in part) by acting as a covalent poison of the human type II enzymes.

Genetic, epigenetic, and lineage-directed mechanisms in benzene-induced malignancies and hematotoxicity targeting hematopoietic stem cells niche

Benzene is a known hematotoxic and leukemogenic agent with hematopoietic stem cells (HSCs) niche being the potential target. Occupational and environmental exposure to benzene has been linked to the incidences of hematological disorders and malignancies. Previous studies have shown that benzene may act via multiple modes of action targeting HSCs niche, which include induction of chromosomal and micro RNA aberrations, leading to genetic and epigenetic modification of stem cells and probable carcinogenesis. However, understanding the mechanism linking benzene to the HSCs niche dysregulation is challenging due to complexity of its microenvironment. The niche is known to comprise of cell populations accounted for HSCs and their committed progenitors of lymphoid, erythroid, and myeloid lineages. Thus, it is fundamental to address novel approaches via lineage-directed strategy to elucidate precise mechanism involved in benzene-induced toxicity targeting HSCs and progenitors of different lineages. Here, we review the key genetic and epigenetic factors that mediate hematotoxicological effects by benzene and its metabolites in targeting HSCs niche. Overall, the use of combined genetic, epigenetic, and lineage-directed strategies targeting the HSCs niche is fundamental to uncover the key mechanisms in benzene-induced hematological disorders and malignancies.

DNA Damage and Perturbed Topoisomerase IIα as a Target of 1,4-Benzoquinone Toxicity in Murine Fetal Liver Cells

Benzene is a ubiquitous environmental pollutant. Recent studies have shown a link between the development of childhood leukemias and maternal benzene exposure, suggesting that these leukemias may be initiated in utero. Benzene crosses the placental barrier however the mechanisms behind in utero benzene toxicity have not been well elucidated. This study is the first to show that the benzene metabolite, benzoquinone (BQ), perturbs fetal topoisomerase IIα (Topo IIα), an enzyme essential for DNA repair. Using cultured murine CD-1 fetal liver cells, this study shows that Topo IIα activity decreases following 24 hours of exposure to BQ (12.5 and 15.625 µM), with the 12.5 µM confirmed to disrupt the c-kit+Lin-Sca-1-Il7rα- population of cells in culture. Pre-treatment with the antioxidant, N-acetylcysteine did not prevent the inhibtion of Topo IIα by BQ. An increase in Topo IIα-DNA covalent adducts was detected following 24-hour exposures to BQ (12.5 and 50 µM). Interestingly, BQ (12.5 µM) exposure did not significantly increase levels of 4-hydroxynonenal (4-HNE), a marker of oxidative stress after 24 hours. However, increased levels of the double-stranded DNA break marker γH2AX were detected following 24 hours of BQ exposure, confirming that Topo IIα-induced breaks are increased in BQ treated cells. This study shows that fetal Topo IIα is perturbed by BQ and suggests that this protein is a target of benzene and may be implicated with in utero benzene toxicity.

DNA Damage by an essential enzyme: A delicate balance act on the tightrope

DNA topoisomerases are essential for DNA metabolic processes such as replication and transcription. Since DNA is double stranded, the unwinding needed for these processes results in DNA supercoiling and catenation of replicated molecules. Changing the topology of DNA molecules to relieve supercoiling or resolve catenanes requires that DNA be transiently cut. While topoisomerases carry out these processes in ways that minimize the likelihood of genome instability, there are several ways that topoisomerases may fail. Topoisomerases can be induced to fail by therapeutic small molecules such as by fluoroquinolones that target bacterial topoisomerases, or a variety of anti-cancer agents that target the eukaryotic enzymes. Increasingly, there have been a large number of agents and processes, including natural products and their metabolites, DNA damage, and the intrinsic properties of the enzymes that can lead to long-lasting DNA breaks that subsequently lead to genome instability, cancer, and other diseases. Understanding the processes that can interfere with topoisomerases and how cells respond when topoisomerases fail will be important in minimizing the consequences when enzymes need to transiently interfere with DNA integrity.

Benzoquinone, a leukemogenic metabolite of benzene, catalytically inhibits the protein tyrosine phosphatase PTPN2 and alters STAT1 signaling

Protein tyrosine phosphatase, nonreceptor type 2 (PTPN2) is mainly expressed in hematopoietic cells, where it negatively regulates growth factor and cytokine signaling. PTPN2 is an important regulator of hematopoiesis and immune/inflammatory responses, as evidenced by loss-of-function mutations of PTPN2 in leukemia and lymphoma and knockout mice studies. Benzene is an environmental chemical that causes hematological malignancies, and its hematotoxicity arises from its bioactivation in the bone marrow to electrophilic metabolites, notably 1,4-benzoquinone, a major hematotoxic benzene metabolite. Although the molecular bases for benzene-induced leukemia are not well-understood, it has been suggested that benzene metabolites alter topoisomerases II function and thereby significantly contribute to leukemogenesis. However, several studies indicate that benzene and its hematotoxic metabolites may also promote the leukemogenic process by reacting with other targets and pathways. Interestingly, alterations of cell-signaling pathways, such as Janus kinase (JAK)/signal transducer and activator of transcription (STAT), have been proposed to contribute to benzene-induced malignant blood diseases. We show here that 1,4-benzoquinone directly impairs PTPN2 activity. Mechanistic and kinetic experiments with purified human PTPN2 indicated that this impairment results from the irreversible formation (k _inact = 645 m^-1·s^-1) of a covalent 1,4-benzoquinone adduct at the catalytic cysteine residue of the enzyme. Accordingly, cell experiments revealed that 1,4-benzoquinone exposure irreversibly inhibits cellular PTPN2 and concomitantly increases tyrosine phosphorylation of STAT1 and expression of STAT1-regulated genes. Our results provide molecular and cellular evidence that 1,4-benzoquinone covalently modifies key signaling enzymes, implicating it in benzene-induced malignant blood diseases.

6,6'-Dihydroxythiobinupharidine as a poison of human type II topoisomerases

A number of natural products with medicinal properties increase DNA cleavage mediated by type II topoisomerases. In an effort to identify additional natural compounds that affect the activity of human type II topoisomerases, a blind screen of a library of 341 Mediterranean plant extracts was conducted. Extracts from Nuphar lutea, the yellow water lily, were identified in this screen. N. lutea has been used in traditional medicine by a variety of indigenous populations. The active compound in N. lutea, 6,6'-dihydroxythiobinupharidine, was found to enhance DNA cleavage mediated by human topoisomerase IIα and IIβ ∼8-fold and ∼3-fold, respectively. Mechanistic studies with topoisomerase IIα indicate that 6,6'-dihydroxythiobinupharidine is a "covalent poison" that acts by adducting the enzyme outside of the DNA cleavage-ligation active site and requires the N-terminal domain of the protein for its activity. Results suggest that some of the medicinal properties of N. lutea may result from the interactions between 6,6'-dihydroxythiobinupharidine and the human type II enzymes.

Concentration-response studies of the chromosome-damaging effects of topoisomerase II inhibitors determined in vitro using human TK6 cells

Topoisomerase II (topo II) inhibitors are commonly used as chemotherapy to treat multiple types of cancer, though their use is also associated with the development of therapy related acute leukemias. While the chromosome-damaging effects of etoposide, a topo II poison, have been proposed to act through a threshold mechanism, little is known about the chromosome damaging effects and dose responses for the catalytic inhibitors of the enzyme. The current study was designed to further investigate the potencies and concentration-response relationships of several topoisomerase II inhibitors, including the topoisomerase II poison etoposide, as well as catalytic inhibitors aclarubicin, merbarone, ICRF-154 and ICRF-187 using both a traditional in vitro micronucleus assay as well as a flow-cytometry based version of the assay. Benchmark dose (BMD) analysis was used to identify models that best fit the data and estimate a BMD, in this case the concentration at which a one standard deviation increase above the control frequency would be expected. All of the agents tested were potent in inducing micronuclei in human lymphoblastoid TK6 cells, with significant increases seen at low micromolar, and in the cases of aclarubicin and etoposide, at low nanomolar concentrations. Use of the anti-kinetochore CREST antibody with the microscopy-based assay demonstrated that the vast majority of the micronuclei originated from chromosome breakage. In comparing the two versions of the micronucleus assay, significant increases in micronucleated cells were observed at similar or lower concentrations using the traditional microscopy-based assay. BMD modeling of the data exhibited several advantages and proved to be a valuable alternative for concentration-response analysis, producing points of departure comparable to those derived using traditional no-observed or lowest-observed genotoxic effect level (NOGEL or LOGEL) approaches.

Role of aneuploidy in the carcinogenic process: Part 3 of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

Aneuploidy is regarded as a hallmark of cancer, however, its role is complex with both pro- and anti-carcinogenic effects evident. In this IWGT review, we consider the role of aneuploidy in cancer biology; cancer risk associated with constitutive aneuploidy; rodent carcinogenesis with known chemical aneugens; and chemotherapy-related malignant neoplasms. Aneuploidy is seen at various stages in carcinogenesis. However, the relationship between induced aneuploidy occurring after exposure and clonal aneuploidy present in tumours is not clear. Recent evidence indicates that the induction of chromosomal instability (CIN), may be more important than aneuploidy per se, in the carcinogenic process. Down Syndrome, trisomy 21, is associated with altered hematopoiesis in utero which, in combination with subsequent mutations, results in an increased risk for acute megakaryoblastic and lymphoblastic leukemias. In contrast, there is reduced cancer risk for most solid tumours in Down Syndrome. Mouse models with high levels of aneuploidy are also associated with increased cancer risk for particular tumours with long latencies, but paradoxically other types of tumour often show decreased incidence. The aneugens reviewed that induce cancer in humans and animals all possess other carcinogenic properties, such as mutagenicity, clastogenicity, cytotoxicity, organ toxicities, hormonal and epigenetic changes which likely account for, or interact with aneuploidy, to cause carcinogenesis. Although the role that aneuploidy plays in carcinogenesis has not been fully established, in many cases, it may not play a primary causative role. Tubulin-disrupting aneugens that do not possess other properties linked to carcinogenesis, were not carcinogenic in rodents. Similarly, in humans, for the tubulin-disrupting aneugens colchicine and albendazole, there is no reported association with increased cancer risk. There is a need for further mechanistic studies on agents that induce aneuploidy, particularly by mechanisms other than tubulin disruption and to determine the role of aneuploidy in pre-neoplastic events and in early and late stage neoplasia.

Aberrant promoter methylation in genes related to hematopoietic malignancy in workers exposed to a VOC mixture

Occupational exposure to volatile organic compounds (VOCs) may cause hematopoietic malignancy, either by single exposure to benzene or possibly due to a concomitant exposure to several VOCs. Since oxidative stress, inflammation and DNA repair pathways are closely involved in cancer development, the effect of VOC exposure on expression of proteins involved in these pathways has been studied, but epigenetic changes have not been well described. Here, DNA methylation status following occupational exposure to a VOC mixture was assessed by bisulfite sequencing of the promoter regions of seven genes involved in the mentioned pathways. Peripheral blood samples and individual-level VOC exposure data were obtained from healthy leather shoe factory workers (LS, n=40) and gas station attendants (GS, n=36), as well as a reference group of university employees (C, n=66). Exposure levels for acetone, ethylbenzene, methyl ethyl ketone, n-hexane, toluene and xylene were higher in LS (p<0.001); benzene and methyl acetate levels were higher in GS (p<0.001). TOP2A, SOD1, and TNF-α promoter methylation status was increased in LS (p<0.05). In LS, we also found significant correlations between GSTP1 promoter methylation and both iNOS (r=0.37, p=0.008) and COX-2 (r=-0.38, p=0.007) methylation. In exposed groups, ethylbenzene exposure levels showed a significant correlation with TOP2A methylation (β=0.33). Our results show early, toxic effects at the epigenetic level caused by occupational exposure to high levels of a VOC mixture. These subcellular modifications may represent the initial mechanism of toxicity leading to hematopoietic malignancy, possibly due to a synergistic, hematotoxic effect of VOC mixtures.

Novel trifluoromethylated 9-amino-3,4-dihydroacridin-1(2H)-ones act as covalent poisons of human topoisomerase IIα

A number of topoisomerase II-targeted anticancer drugs, including amsacrine, utilize an acridine or related aromatic core as a scaffold. Therefore, to further explore the potential of acridine-related compounds to act as topoisomerase II poisons, we synthesized a series of novel trifluoromethylated 9-amino-3,4-dihydroacridin-1(2H)-one derivatives and examined their ability to enhance DNA cleavage mediated by human topoisomerase IIα. Derivatives containing a H, Cl, F, and Br at C7 enhanced enzyme-mediated double-stranded DNA cleavage ∼5.5- to 8.5-fold over baseline, but were less potent than amsacrine. The inclusion of an amino group at C9 was critical for activity. The compounds lost their activity against topoisomerase IIα in the presence of a reducing agent, displayed no activity against the catalytic core of topoisomerase IIα, and inhibited DNA cleavage when incubated with the enzyme prior to the addition of DNA. These findings strongly suggest that the compounds act as covalent, rather than interfacial, topoisomerase II poisons.

The role of urinary pH in o-phenylphenol-induced cytotoxicity and chromosomal damage in the bladders of F344 rats

o-Phenylphenol (OPP) is a widely used fungicide and antibacterial agent that at high doses has been shown to cause bladder cancer in male F344 rats. The mechanisms underlying OPP-induced bladder carcinogenicity remain unclear but it has been proposed that a non-enzymatic pH-dependent autoxidation of phenylhydroquinone (PHQ), a primary metabolite of OPP, may be a key step in OPP-induced rat bladder carcinogenesis. To investigate this mechanism and to provide insights into the potential human health relevance of OPP-induced cancer, a series of in vitro and in vivo experiments were conducted. In human lymphoblastoid TK-6 cells and rat bladder epithelial NBT-II cells, strong increases in cytotoxicity were seen at a constant concentration of PHQ by increasing the buffer pH as well as by increasing concentrations of PHQ at a constant pH. In in vivo studies, male rats were administered OPP (4,000 and 8,000 ppm) in a diet supplemented with either 1% ammonium chloride or 3% sodium bicarbonate to produce acidic and alkaline urinary pH, respectively. Significant increases in cell proliferation as detected by 5-bromo-2'-deoxyuridine incorporation and micronucleus formation were seen in the bladder cells of OPP-treated rats with neutral or alkaline urinary pH but not in animals with the acidified urine. The results from these in vitro and in vivo studies provide support for the autoxidation hypothesis of bioactivation, and provide additional evidence that urinary pH can significantly influence the genotoxicity and carcinogenicity of this important agent.

Effects of Secondary Metabolites from the Fungus Septofusidium berolinense on DNA Cleavage Mediated by Human Topoisomerase IIα

Two metabolites from the ascomycete fungus Septofusidium berolinense were recently identified as having antineoplastic activity [Ekiz et al. (2015) J. Antibiot. , DOI: 10.1038/ja.2015.84]. However, the basis for this activity is not known. One of the compounds [3,6-dihydroxy-2-propylbenzaldehyde (GE-1)] is a hydroquinone, and the other [2-hydroxymethyl-3-propylcyclohexa-2,5-diene-1,4-dione (GE-2)] is a quinone. Because some hydroquinones and quinones act as topoisomerase II poisons, the effects of GE-1 and GE-2 on DNA cleavage mediated by human topoisomerase IIα were assessed. GE-2 enhanced DNA cleavage ∼4-fold and induced scission with a site specificity similar to that of the anticancer drug etoposide. Similar to other quinone-based topoisomerase II poisons, GE-2 displayed several hallmark characteristics of covalent topoisomerase II poisons, including (1) the inability to poison a topoisomerase IIα construct that lacks the N-terminal domain, (2) the inhibition of DNA cleavage when the compound was incubated with the enzyme prior to the addition of plasmid, and (3) the loss of poisoning activity in the presence of a reducing agent. In contrast to GE-2, GE-1 did not enhance DNA cleavage mediated by topoisomerase IIα except at very high concentrations. However, the activity and potency of the metabolite were dramatically enhanced under oxidizing conditions. These results suggest that topoisomerase IIα may play a role in mediating the cytotoxic effects of these fungal metabolites.

Effects of Olive Metabolites on DNA Cleavage Mediated by Human Type II Topoisomerases

Several naturally occurring dietary polyphenols with chemopreventive or anticancer properties are topoisomerase II poisons. To identify additional phytochemicals that enhance topoisomerase II-mediated DNA cleavage, a library of 341 Mediterranean plant extracts was screened for activity against human topoisomerase IIα. An extract from Phillyrea latifolia L., a member of the olive tree family, displayed high activity against the human enzyme. On the basis of previous metabolomics studies, we identified several polyphenols (hydroxytyrosol, oleuropein, verbascoside, tyrosol, and caffeic acid) as potential candidates for topoisomerase II poisons. Of these, hydroxytyrosol, oleuropein, and verbascoside enhanced topoisomerase II-mediated DNA cleavage. The potency of these olive metabolites increased 10–100-fold in the presence of an oxidant. Hydroxytyrosol, oleuropein, and verbascoside displayed hallmark characteristics of covalent topoisomerase II poisons. (1) The activity of the metabolites was abrogated by a reducing agent. (2) Compounds inhibited topoisomerase II activity when they were incubated with the enzyme prior to the addition of DNA. (3) Compounds were unable to poison a topoisomerase IIα construct that lacked the N-terminal domain. Because hydroxytyrosol, oleuropein, and verbascoside are broadly distributed across the olive family, extracts from the leaves, bark, and fruit of 11 olive tree species were tested for activity against human topoisomerase IIα. Several of the extracts enhanced enzyme-mediated DNA cleavage. Finally, a commercial olive leaf supplement and extra virgin olive oils pressed from a variety of Olea europea subspecies enhanced DNA cleavage mediated by topoisomerase IIα. Thus, olive metabolites appear to act as topoisomerase II poisons in complex formulations intended for human dietary consumption.

Heck J, Malagoli C, Del Giovane C, Vinceti M. A review and meta-analysis of outdoor air pollution and risk of childhood leukemia

Leukemia is the most frequent malignant disease affecting children. To date, the etiology of childhood leukemia remains largely unknown. Few risk factors (genetic susceptibility, infections, ionizing radiation, etc.) have been clearly identified, but they appear to explain only a small proportion of cases. Considerably more uncertain is the role of other environmental risk factors, such as indoor and outdoor air pollution. We sought to summarize and quantify the association between traffic-related air pollution and risk of childhood leukemia, and further examined results according to method of exposure assessment, study quality, leukemia subtype, time period, and continent where studies took place. After a literature search yielded 6 ecologic and 20 case-control studies, we scored the studies based on the Newcastle-Ottawa Scale. The studies assessed residential exposure to pollutants from motorized traffic by computing traffic density in the neighboring roads or vicinity to petrol stations, or by using measured or modeled nitrogen dioxide and benzene outdoor air levels. Because heterogeneity across studies was observed, random-effects summary odds ratios (OR) and 95% confidence intervals (CI) were reported. Whenever possible we additionally conducted stratified analyses comparing acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Limiting the analysis to high-quality studies (Newcastle-Ottawa Scale ≥ 7), those using traffic density as the exposure assessment metric showed an increase in childhood leukemia risk in the highest exposure category (OR = 1.07, 95% CI 0.93-1.24). However, we observed evidence of publication bias. Results for NO2 exposure and benzene showed an OR of 1.21 (95% CI 0.97-1.52) and 1.64 (95% CI 0.91-2.95) respectively. When stratifying by leukemia type, the results based upon NO2 were 1.21 (95% CI 1.04-1.41) for ALL and 1.06 (95% CI 0.51-2.21) for AML; based upon benzene were 1.09 (95% CI 0.67-1.77) for ALL and 2.28 (95% CI 1.09-4.75) for AML. Estimates were generally higher for exposures in the postnatal period compared to the prenatal period, and for European studies compared to North American studies. Overall, our results support a link between ambient exposure to traffic pollution and childhood leukemia risk, particularly due to benzene.

Catalytic core of human topoisomerase IIα: insights into enzyme-DNA interactions and drug mechanism

Coordination between the N-terminal gate and the catalytic core of topoisomerase II allows the proper capture, cleavage, and transport of DNA during the catalytic cycle. Because the activities of these domains are tightly linked, it has been difficult to discern their individual contributions to enzyme-DNA interactions and drug mechanism. To further address the roles of these domains, we analyzed the activity of the catalytic core of human topoisomerase IIα. The catalytic core and the wild-type enzyme both maintained higher levels of cleavage with negatively (as compared to positively) supercoiled plasmid, indicating that the ability to distinguish supercoil handedness is embedded within the catalytic core. However, the catalytic core alone displayed little ability to cleave DNA substrates that did not intrinsically provide the enzyme with a transport segment (i.e., substrates that did not contain crossovers). Finally, in contrast to interfacial topoisomerase II poisons, covalent poisons did not enhance DNA cleavage mediated by the catalytic core. This distinction allowed us to further characterize the mechanism of etoposide quinone, a drug metabolite that functions primarily as a covalent poison. Etoposide quinone retained some ability to enhance DNA cleavage mediated by the catalytic core, indicating that it still can function as an interfacial poison. These results further define the distinct contributions of the N-terminal gate and the catalytic core to topoisomerase II function. The catalytic core senses the handedness of DNA supercoils during cleavage, while the N-terminal gate is critical for capturing the transport segment and for the activity of covalent poisons.

Etoposide quinone is a covalent poison of human topoisomerase IIβ

Etoposide is a topoisomerase II poison that is utilized to treat a broad spectrum of human cancers. Despite its wide clinical use, 2–3% of patients treated with etoposide eventually develop treatment-related acute myeloid leukemias (t-AMLs) characterized by rearrangements of the MLL gene. The molecular basis underlying the development of these t-AMLs is not well understood; however, previous studies have implicated etoposide metabolites (i.e., etoposide quinone) and topoisomerase IIβ in the leukemogenic process. Although interactions between etoposide quinone and topoisomerase IIα have been characterized, the effects of the drug metabolite on the activity of human topoisomerase IIβ have not been reported. Thus, we examined the ability of etoposide quinone to poison human topoisomerase IIβ. The quinone induced ∼4 times more enzyme-mediated DNA cleavage than did the parent drug. Furthermore, the potency of etoposide quinone was ∼2 times greater against topoisomerase IIβ than it was against topoisomerase IIα, and the drug reacted ∼2–4 times faster with the β isoform. Etoposide quinone induced a higher ratio of double- to single-stranded breaks than etoposide, and its activity was less dependent on ATP. Whereas etoposide acts as an interfacial topoisomerase II poison, etoposide quinone displayed all of the hallmarks of a covalent poison: the activity of the metabolite was abolished by reducing agents, and the compound inactivated topoisomerase IIβ when it was incubated with the enzyme prior to the addition of DNA. These results are consistent with the hypothesis that etoposide quinone contributes to etoposide-related leukemogenesis through an interaction with topoisomerase IIβ.

Lymphohematopoietic cancers induced by chemicals and other agents and their implications for risk evaluation: An overview

Lymphohematopoietic neoplasia are one of the most common types of cancer induced by therapeutic and environmental agents. Of the more than 100 human carcinogens identified by the International Agency for Research on Cancer, approximately 25% induce leukemias or lymphomas. The objective of this review is to provide an introduction into the origins and mechanisms underlying lymphohematopoietic cancers induced by xenobiotics in humans with an emphasis on acute myeloid leukemia, and discuss the implications of this information for risk assessment. Among the agents causing lymphohematopoietic cancers, a number of patterns were observed. Most physical and chemical leukemia-inducing agents such as the therapeutic alkylating agents, topoisomerase II inhibitors, and ionizing radiation induce mainly acute myeloid leukemia through DNA-damaging mechanisms that result in either gene or chromosomal mutations. In contrast, biological agents and a few immunosuppressive chemicals induce primarily lymphoid neoplasms through mechanisms that involve alterations in immune response. Among the environmental agents examined, benzene was clearly associated with acute myeloid leukemia in humans, with increasing but still limited evidence for an association with lymphoid neoplasms. Ethylene oxide and 1,3-butadiene were linked primarily to lymphoid cancers. Although the association between formaldehyde and leukemia remains controversial, several recent evaluations have indicated a potential link between formaldehyde and acute myeloid leukemia. The four environmental agents examined in detail were all genotoxic, inducing gene mutations, chromosomal alterations, and/or micronuclei in vivo. Although it is clear that rapid progress has been made in recent years in our understanding of leukemogenesis, many questions remain for future research regarding chemically induced leukemias and lymphomas, including the mechanisms by which the environmental agents reviewed here induce these diseases and the risks associated with exposures to such agents.

Natural products as topoisomerase II poisons: effects of thymoquinone on DNA cleavage mediated by human topoisomerase IIα

The seeds of Nigella sativa (often referred to as black seed) have long been utilized as a medicinal herb in Middle Eastern, Northern African, and Indian cultures. Historically, black seed has been used to treat a variety of illnesses associated with inflammation. More recent studies have found that it induces apoptosis and displays anticancer activity in animal and cellular models. The major bioactive compound of black seed is thymoquinone, which shares structural features with 1,4-benzoquinone and other covalent topoisomerase II poisons. Because a number of anticancer drugs target type II topoisomerases, we determined the effects of thymoquinone and a series of related quinones on human topoisomerase IIα. Thymoquinone enhanced enzyme-mediated DNA cleavage ∼5-fold, which is similar to the increase seen with the anticancer drug etoposide. In order to enhance cleavage, compounds had to have at least two positions available for acylation. Furthermore, activity was decreased by the inclusion of electron-donating groups or bulky substituents. As predicted for a covalent topoisomerase II poison, the activity of thymoquinone (and related compounds) was abrogated by the addition of a reducing agent. Also, thymoquinone inhibited topoisomerase IIα activity when incubated with the enzyme prior to the addition of DNA. Cleavage complexes formed in the presence of the compound were stable for at least 8 h. Lastly, black seed extract and black seed oil both increased levels of enzyme-mediated DNA cleavage, suggesting that thymoquinone is active even in more complex herbal formulations. These findings indicate that thymoquinone can be added to the growing list of dietary and medicinal natural products with activity against human type II topoisomerases.

Redox state alters anti-cancer effects of wedelolactone

Wedelolactone is one of the active plant polyphenolic compounds. Anti-tumor effects of this drug have been demonstrated recently. We have described that wedelolactone acts as catalytic inhibitor of DNA topoisomerase IIα. The aim of this study was to further characterize the mechanism of its anti-tumor effects. We showed that wedelolactone inhibits binding of DNA topoisomerase IIα to plasmid DNA and antagonizes formation of etoposide-induced DNA cleavage complex. The inhibition of topoisomerase IIα by wedelolactone is reversible by excess of the enzyme but not DNA. The in vitro inhibitory effect of wedelolactone on the topoisomerase IIα activity is redox-dependent as it diminished in the presence of reducing agents. Cytotoxicity of wedelolactone was partially inhibited by N-acetylcysteine and glutathione ethyl ester in breast cancer MDA-MB-231 and MDA-MB-468 cells while the inhibitory effect of catalase was observed only in the former cell line. Finally, we found that wedelolactone can be oxidized in the presence of copper ions resulting in DNA strand break and abasic site formation in vitro. However, wedelolactone induced neither DNA damage in MDA-MB-231 cells nor mutations in bacterial cells detectable by Ames test suggesting that wedelolactone may not be an effective inducer of DNA damage. We conclude that the topoisomerase IIα inhibitory- and DNA damaging activities of wedelolactone in vitro depend on its redox state. Pro-oxidant activity could, however, explain only part of wedelolactone-induced cytotoxicity. Therefore, the major cellular target(s) of wedelolactone and the exact mechanism of wedelolactone-induced cytotoxicity still remain to be identified.

Mechanism of inhibition of the ATPase domain of human topoisomerase IIα by 1,4-benzoquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, and 9,10-phenanthroquinone

The inhibition of human topoisomerase IIα (Hu-TopoIIα), a major enzyme involved in maintaining DNA topology, repair, and chromosome condensation/decondensation results in loss of genomic integrity. In the present study, the inhibition of ATPase domain of Hu-TopoIIα as a possible mechanism of genotoxicity of 1,4-benzoquinone (BQ), hydroquinone (HQ), naphthoquinone (1,2-NQ and 1,4-NQ), and 9,10-phenanthroquinone (9,10-PQ) was investigated. In silico modeling predicted that 1,4-BQ, 1,2-NQ, 1,4-NQ, and 9,10-PQ could interact with Ser-148, Ser-149, Asn-150, and Asn-91 residues of the ATPase domain of Hu-TopoIIα. Biochemical inhibition assays with the purified ATPase domain of Hu-TopoIIα revealed that 1,4-BQ is the most potent inhibitor followed by 1,4-NQ > 1,2-NQ > 9,10-PQ > HQ. Ligand-binding studies using isothermal titration calorimetry revealed that 1,4-BQ, HQ, 1,4-NQ, 1,2-NQ, and 9,10-PQ enter into four sequentially binding site models inside the domain. 1,4-BQ exhibited the strongest binding, followed by 1,4-NQ > 1,2-NQ > 9,10-PQ > HQ, as revealed by their average K(d) values. The cellular fate of such inhibition was further evidenced by an increase in the number of Hu-TopoIIα-DNA cleavage complexes in the human lung epithelial cells (BEAS-2B) using trapped in agarose DNA immunostaining (TARDIS) assay, which utilizes antibody specific for Hu-TopoIIα. Furthermore, the increase in γ-H2A.X levels quantitated by flow cytometry and visualized by immunofluorescence microscopy illustrated that accumulation of DNA double-strand breaks inside the cells can be attributed to the inhibition of Hu-TopoIIα. These findings collectively suggest that 1,4-BQ, 1,2-NQ, 1,4-NQ, and 9,10-PQ inhibit the ATPase domain and potentially result in Hu-TopoIIα-mediated clastogenic and leukemogenic events.

Bimolane induces multiple types of chromosomal aberrations in human lymphocytes in vitro

Bimolane has been commonly used in China for the treatment of psoriasis and various types of cancer. Patients treated with bimolane have been reported to have an increased risk of developing therapy-related leukemias. Although bimolane has been identified as a human leukemia-inducing agent, little is known about its genotoxic effects, and a systematic study of the types of chromosomal alterations induced by this compound has not been performed. In this study, a combination of immunochemical, molecular and conventional cytogenetic techniques has been used to study the chromosomal alterations induced by bimolane in cultured human lymphocytes. Immunochemical staining with the CREST antibody indicated that bimolane induces micronuclei (MN) originating primarily from chromosome breakage. Interestingly fluorescence in situ hybridization (FISH) with differentially labeled chromosomes 1 and 9 centromeric probes indicated that bimolane also caused non-disjunction and polyploidy. Consistent with this, an expedited analysis of Giemsa-stained metaphase chromosomes in bimolane-treated lymphocytes revealed a high frequency of polyploidy/hyperdiploidy as well as dicentric chromosomes, and premature centromeric division (PCD). In addition, bimolane was also found to produce binucleated cells, possibly through an interference with normal functioning of intermediate filaments. As a follow-up to these studies, three different types of commercially available bimolane formulations obtained from different Chinese manufacturers were also evaluated. The effects seen with the formulated bimolane were similar to those seen with the synthesized compound. Our studies indicate that bimolane effectively induces a variety of cellular and chromosomal changes in cultured lymphocytes and that similar alterations occurring in bone marrow stem cells could contribute to the development of the secondary cancers seen in bimolane-treated patients.

Cellular damages in the Allium cepa test system, caused by BTEX mixture prior and after biodegradation process

Petroleum and derivatives have been considered one of the main environmental contaminants. Among petroleum derivatives, the volatile organic compounds benzene, toluene, ethylbenzene and xylene (BTEX) represent a major concern due to their toxicity and easy accumulation in groundwater. Biodegradation methods seem to be suitable tools for the clean-up of BTEX contaminants from groundwater. Genotoxic and mutagenic potential of BTEX prior and after biodegradation process was evaluated through analyses of chromosomal aberrations and MN test in meristematic and F(1) root cells using the Allium cepa test system. Seeds of A. cepa were germinated into five concentrations of BTEX, non-biodegraded and biodegraded, in ultra-pure water (negative control), in MMS 4×10(-4)M (positive control) and in culture medium used in the biodegradation (blank biodegradation control). Results showed a significant frequency of both chromosomal and nuclear aberrations. The micronucleus (MN) frequency in meristematic cells was significant for most of tested samples. However, MN was not present in significant levels in the F(1) cells, suggesting that there was no permanent damage for the meristematic cell. The BTEX effects were significantly reduced in the biodegraded samples when compared to the respective non-biodegraded concentrations. Therefore, in this study, the biodegradation process showed to be a reliable and effective alternative to treat BTEX-contaminated waters. Based on our results and available data, the BTEX toxicity could also be related to a synergistic effect of its compounds.

Maternal exposure to household chemicals and risk of infant leukemia: a report from the Children's Oncology Group

OBJECTIVE

Utilizing data from the largest study to date, we examined associations between maternal preconception/prenatal exposure to household chemicals and infant acute leukemia.

METHODS

We present data from a Children's Oncology Group case-control study of 443 infants (<1 year of age) diagnosed with acute leukemia [including acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML)] between 1996 and 2006 and 324 population controls. Mothers recalled household chemical use 1 month before and throughout pregnancy. We used unconditional logistic regression adjusted for birth year, maternal age, and race/ethnicity to calculate odds ratios (ORs) and 95% confidence intervals (CIs).

RESULTS

We did not find evidence for an association between infant leukemia and eight of nine chemical categories. However, exposure to petroleum products during pregnancy was associated with AML (OR = 2.54; 95% CI:1.40-4.62) and leukemia without mixed lineage leukemia (MLL) gene rearrangements ("MLL-") (OR = 2.69; 95% CI: 1.47-4.93). No associations were observed for exposure in the month before pregnancy.

CONCLUSIONS

Gestational exposure to petroleum products was associated with infant leukemia, particularly AML, and MLL- cases. Benzene is implicated as a potential carcinogen within this exposure category, but a clear biological mechanism has yet to be elucidated.