Mutagenicity and cytotoxicity of reactive oxygen and nitrogen species in the MN-11 murine tumor cell line

Adverse outcome pathways for ionizing radiation and breast cancer involve direct and indirect DNA damage, oxidative stress, inflammation, genomic instability, and interaction with hormonal regulation of the breast

Knowledge about established breast carcinogens can support improved and modernized toxicological testing methods by identifying key mechanistic events. Ionizing radiation (IR) increases the risk of breast cancer, especially for women and for exposure at younger ages, and evidence overall supports a linear dose-response relationship. We used the Adverse Outcome Pathway (AOP) framework to outline and evaluate the evidence linking ionizing radiation with breast cancer from molecular initiating events to the adverse outcome through intermediate key events, creating a qualitative AOP. We identified key events based on review articles, searched PubMed for recent literature on key events and IR, and identified additional papers using references. We manually curated publications and evaluated data quality. Ionizing radiation directly and indirectly causes DNA damage and increases production of reactive oxygen and nitrogen species (RONS). RONS lead to DNA damage and epigenetic changes leading to mutations and genomic instability (GI). Proliferation amplifies the effects of DNA damage and mutations leading to the AO of breast cancer. Separately, RONS and DNA damage also increase inflammation. Inflammation contributes to direct and indirect effects (effects in cells not directly reached by IR) via positive feedback to RONS and DNA damage, and separately increases proliferation and breast cancer through pro-carcinogenic effects on cells and tissue. For example, gene expression changes alter inflammatory mediators, resulting in improved survival and growth of cancer cells and a more hospitable tissue environment. All of these events overlap at multiple points with events characteristic of "background" induction of breast carcinogenesis, including hormone-responsive proliferation, oxidative activity, and DNA damage. These overlaps make the breast particularly susceptible to ionizing radiation and reinforce that these biological activities are important characteristics of carcinogens. Agents that increase these biological processes should be considered potential breast carcinogens, and predictive methods are needed to identify chemicals that increase these processes. Techniques are available to measure RONS, DNA damage and mutation, cell proliferation, and some inflammatory proteins or processes. Improved assays are needed to measure GI and chronic inflammation, as well as the interaction with hormonally driven development and proliferation. Several methods measure diverse epigenetic changes, but it is not clear which changes are relevant to breast cancer. In addition, most toxicological assays are not conducted in mammary tissue, and so it is a priority to evaluate if results from other tissues are generalizable to breast, or to conduct assays in breast tissue. Developing and applying these assays to identify exposures of concern will facilitate efforts to reduce subsequent breast cancer risk.

Mesenchymal Stem Cell Treatment of Inflammation-Induced Cancer

Cancer development is often associated with chronic inflammation. To date, research into inflammation-induced cancer has largely focused on chemokines, cytokines, and their downstream targets. These inflammatory mediators may promote tumor growth, invasion, metastasis, and facilitate angiogenesis. However, the exact mechanisms by which inflammation promotes neoplasia remain unclear. Inflammatory bowel disease (IBD) is characterized by recurrent, idiopathic intestinal inflammation, the complications of which are potentially fatal. IBD incidence in Australia is 24.2 per 100,000 and its peak onset is in people aged 15 to 24 years. Symptoms include abdominal pain, cramps, bloody stool, and persistent diarrhoea or constipation and so seriously compromise quality of life. However, due to its unknown etiology, current treatment strategies combat the symptoms rather than the disease and are limited by inefficacy, toxicity, and adverse side-effects. IBD is also associated with an increased risk of colorectal cancer, for which treatment options are similarly limited. In recent years, there has been much interest in the therapeutic potential of mesenchymal stem cells (MSCs). However, whether MSCs suppress or promote tumor development is still contentious within the literature. Many studies indicate that MSCs exert anti-tumor effects and suppress tumor growth, whereas other studies report pro-tumor effects. Studies using MSCs as treatment for IBD have shown promising results in both animal models and human trials. However, as MSC treatment is still novel, the long-term risks remain unknown. This review aims to summarize the current literature on MSC treatment of inflammation-induced cancer, with a focus on colorectal cancer resulting from IBD.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Chromosomal aberrations in arsenic-exposed human populations: a review with special reference to a comprehensive study in West Bengal, India

For centuries arsenic has played an important role in science, technology, and medicine. Arsenic for its environmental pervasiveness has gained unexpected entrance to the human body through food, water and air, thereby posing a great threat to public health due to its toxic effect and carcinogenicity. Thus, in modern scenario arsenic is synonymous with "toxic" and is documented as a paradoxical human carcinogen, although its mechanism of induction of neoplasia remains elusive. To assess the risk from environmental and occupational exposure of arsenic, in vivo cytogenetic assays have been conducted in arseniasis-endemic areas of the world using chromosomal aberrations (CA) and sister chromatid exchanges (SCE) as biomarkers in peripheral blood lymphocytes. The primary aim of this report is to critically review and update the existing in vivo cytogenetic studies performed on arsenic-exposed populations around the world and compare the results on CA and SCE from our own study, conducted in arsenic-endemic villages of North 24 Parganas (district) of West Bengal, India from 1999 to 2003. Based on a structured questionnaire, 165 symptomatic (having arsenic induced skin lesions) subjects were selected as the exposed cases consuming water having a mean arsenic content of 214.96 microg/l. For comparison 155 age-sex matched control subjects from an unaffected district (Midnapur) of West Bengal were recruited. Similar to other arsenic exposed populations our population also showed a significant difference (P < 0.01) in the frequencies of CA and SCE between the cases and control group. Presence of substantial chromosome damage in lymphocytes in the exposed population predicts an increased future carcinogenic risk by this metalloid.

Urinary bladder lesions induced by persistent chronic low-dose ionizing radiation

The incidence of urinary bladder cancer in the Ukraine increased from 26.2 to 43.3 per 100,000 population between 1986 and 2001 after the Chernobyl accident. The present study was conducted to evaluate the development of radiation-dependent lesions in the urinary bladders of people living in cesium 137 ((137)Cs) radio-contaminated areas of the Ukraine. Bladder urothelial biopsies from 159 male and 5 female patients were subjected to histological examination and immunohistochemical study of p38 mitogen-activated protein kinase (MAPK), as well as the p50 and p65 subunits of nuclear factor kappa B (NF-kappa B). A pattern of chronic proliferative atypical cystitis accompanied with large areas of sclerosis of connective tissue in the lamina propria was commonly observed in all cases. Interestingly, these lesions were associated with a dramatic increase in the incidences of dysplasia/carcinoma in situ, and, moreover, small urothelial carcinomas were incidentally detected. We defined the overall condition as "Chernobyl cystitis." Greatly elevated levels of p38, p65 and p50 expression in the urothelium were evident and the patients showed increased (137)Cs in urine. The data support conclusions from our previous studies of a critical role for increased oxidative stress in generation of urinary bladder urothelial lesions in individuals chronically exposed to low-dose (137)Cs radiation. Alterations in the p38 MAPK cascade and accumulation of NF-kappa B subunits could be crucial early molecular events in the pathogenesis of Chernobyl cystitis.

Selective nitration of histone tyrosine residues in vivo in mutatect tumors

Nitric oxide-derived reactive species have been implicated in many disorders. Protein nitrotyrosine is often used as a stable marker of these reactive species. Using immunohistochemistry, we have previously detected nitrotyrosine in murine Mutatect tumors, where neutrophils are the principal source of nitric oxide. We now report on the identification of several prominent nitrotyrosine-containing proteins. Using Western blot analysis, nitrotyrosine in higher molecular mass proteins (>20 kDa) was detected in tumors containing a high number of neutrophils but not in tumors with fewer neutrophils. Staining for nitrotyrosine was consistently seen in low molecular mass proteins (< or =15 kDa), regardless of the level of neutrophils. Protein nitrotyrosine was not seen in Mutatect cells growing in vitro. Treatment with nitric oxide donors produced nitration of < or =15-kDa proteins, but only after extended periods. These small proteins, both from tumors and cultured cells, were identified by mass spectrometry to be histones. Only a subset of tyrosine residues was nitrated. Selective nitration may reflect differential accessibility of different tyrosine residues and the influence of neighboring residues within the nucleosome. The prominence of histone nitration may reflect its relative stability, making this post-translational modification a potentially useful marker of extended exposure of cells or tissues to nitric oxide-derived reactive species.

The genetics of the target tissue in rheumatoid arthritis

The genetic mechanisms that are complementary in predisposing and then establishing disease are yet to be fully elucidated. During a lifetime, the genetic composition of the host is not only hereditary but undergoes rearrangements, integrations, and more subtle single-base pair alterations. These changes can be inconsequential or lead to aberrant and deleterious pathologic changes. In a complex multifactorial disease such as RA, the relative roles of the dynamic versus germline elements of the disease have yet to be fully determined. Further studies of large populations are likely to segregate out factors affecting specific ethnic, clinical, and genetic subgroups.

Effect of hydrogen peroxide on nitric oxide (NO)-induced mutagenicity in Salmonella typhimurium

Nitric oxide (NO) has been reported to impart, alone or in combination with reactive oxygen species (ROS), the cytotoxicity and putative genotoxicity associated with the immunological response. The present study examined the change in the mutagenic activity profile of the NO-donor spermine NONOate (SperNO) as a result of introduction of hydrogen peroxide (H(2)O(2)) to the Ames assay. The aim was to determine whether the assay could detect H(2)O(2)-induced co- or anti-mutagenic effects on NO-induced mutagenesis, and the Salmonella typhimurium base-pair substitution tester strain TA1535 provided an appropriate tool. While TA1535 was shown by the authors and others to be strongly sensitive to NO-induced mutagenesis, it has also been shown to be insensitive to H(2)O(2)-induced mutagenicity [1,2]. When H(2)O(2) (0.25-4.0 micromol/pl) was added directly to cells treated with SperNO (0.01-1.0 micromol/pl), co-mutagenicity was not detected, but a drop in reversion count and detectable toxicity was observed, especially at doses > 0.1 micromol/pl. When glucose/glucose oxidase (GOX) or reduced glutathione (GSH) were used as H(2)O(2)-generation systems the results varied. Reversion induced by SperNO (1 micromol/pl) was moderately enhanced by GOX (10-20 mUnits/pl), but the increase albeit reproducible did not reach a doubling (co-mutagenicity). GOX (40 micromol/pl) induced a reduction in reversion count, but no visible toxicity. On the other hand, GSH (20- 80 micromol/pl) gave a strong co-mutagenic effect. Co-mutagenicity was highest (> 5x) at 80 micromol/pl GSH and 0.1 micromol/pl SperNO. Based on these findings, it could be concluded that a) H(2)O(2), when steadily generated in the cell, has a modulatory effect on NO-mutagenicity, and such a conclusion is not inconsistent with the wide range of responses reported for the two chemicals, and/or b) the observed co-mutagenic effects of GSH may not be attributable solely to H(2)O(2) generation.

Genotoxicity of streptonigrin: a review

Streptonigrin (SN, CAS no. 3930-19-6) is an aminoquinone antitumor antibiotic isolated from cultures of Streptomyces flocculus. This compound is a member of a group of antitumor agents which possess the aminoquinone moiety and that includes also mitomycin C, porfiromycin, actinomycin, rifamycin and geldanamycin. Because of the potential use of SN in clinical chemotherapy, the study of its genotoxicity has considerable practical significance.SN inhibits the synthesis of DNA and RNA, causes DNA strand breaks after reduction with NADH, induces unscheduled DNA synthesis and DNA adducts and inhibits topoisomerase II. At the chromosome level, this antibiotic causes chromosome damage and increases the frequency of sister-chromatid exchanges.SN cleaves DNA in cell-free systems by a mechanism that involves complexing with metal ions and autoxidation of the quinone moiety to semiquinone in the presence of NADH with production of oxygen-derived reactive species. Recent evidence strongly suggests that the clastogenic action of this compound is partially mediated by free radicals. The present review aims at summarizing past and current knowledge concerning the genotoxic effects of SN.

Uridine uptake inhibition as a cytotoxicity test for a human hepatoma cell line (HepG2 cells): comparison with the neutral red assay

This study describes a sensitive microassay for measuring cytotoxicity based on the degree of inhibition of RNA synthesis in HepG2 cells. RNA synthesis is measured by the kinetic uptake of radiolabeled uridine. A large number of compounds were tested in a wide range of concentrations. The concentration required to induce 50% inhibition of HepG2 uridine uptake rates (IC(50)) was determined for each compound and used to rank its potency. These IC(50)s were compared with IC(50)s measured with the neutral red assay. 2-acetylaminofluorene, benzo[a]pyrene and methylnitrosourea were not cytotoxic in the neutral red assay. Uridine uptake was always inhibited at lower concentrations than those required in the neutral red assay, suggesting that the uridine uptake assay is a more sensitive indicator of toxic action than the neutral red inclusion. Uridine uptake assay provides a rapid and quantitative method for assessing toxicity in a human cell line. Application of this method to bottled spring waters are described. Due to its high sensitivity and reproducibility, this method provides a suitable tool for screening a great number of samples and will be a helpful test for evaluating food safety and controlling the recycling process of wrapping materials.

Constitutive expression of interleukin-8 by Mutatect cells markedly affects their tumor biology

Interleukin-8 (IL-8) is a chemokine for neutrophils and an angiogenic factor. Human tumors that express IL-8 may exhibit intense neutrophil infiltration and increased vascularization. Mutatect cells are a murine fibrosarcoma that can be grown as subcutaneous tumors in syngeneic C57BL/6 mice. Since neutrophils are a source of cytotoxic and genotoxic species, we constructed Mutatect cell lines that constitutively express human IL-8 to explore the involvement of neutrophils in tumor biology and genetic instability. An IL-8/neo expression plasmid was stably transfected into Mutatect MC17-51 cells and clone MIL-4 was isolated. Tumors initiated with 5x10(5) MIL-4 cells grew very slowly compared with tumors from pure MC17-51 cells or from 0.5 to 4x10(5) MIL-4 cells mixed with 5x10(5) MC17-51 cells. Over 95% of cells recovered from slow-growing pure MIL-4 tumors lost the transgene as measured by loss of (i) resistance to G418, (ii) expression of IL-8 protein and (iii) IL-8-specific DNA sequences. When tumors from mixed cell types were examined, loss of the transgene did not occur; rather, IL-8 producing cells appeared to have some growth advantage. The neutrophil content of tumors (as measured by myeloperoxidase) was directly proportional to the level of IL-8 expressed at the time tumors were excised. As reported earlier, the frequency of mutations at the hypoxanthine phosphoribosyltransferase locus was also directly proportional to neutrophil content. To explain some of these biological findings, we postulate that early in development of pure MIL-4 tumors, genotoxic/cytotoxic neutrophils are attracted by IL-8, which in turn leads to loss of the transgene and to localized cytotoxicity of IL-8 producing cells. In mixed tumors, where the initial IL-8 concentration may be lower, tumors might become established more readily because fewer neutrophils may be attracted. This relatively simple experimental paradigm has revealed some of the complex biological changes that can occur as a result of IL-8 in tumors.

Role of calcium in nitric oxide-induced cytotoxicity: EGTA protects mouse oligodendrocytes

Active nitrogen species are overproduced in inflammatory brain lesions in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). NO has been shown to mediate the death of oligodendrocytes (OLs), a primary target of damage in MS. To develop strategies to protect OLs, we examined the mechanisms of cytotoxicity of two NO donors, S-nitroso-N-acetyl-penicillamine (SNAP) and sodium nitroprusside (SNP) on mature mouse OLs. Nitrosonium ion (NO+) rather than NO. mediates damage with both SNAP and SNP, as shown by significant protection with hemoglobin (HbO2), but not with the NO. scavenger PTIO. SNAP and SNP differ in time course and mechanisms of killing OLs. With SNAP, OL death is delayed for at least 6 hr, but with SNP, OL death is continuous over 18 hr with no delay. Relative to NO release, SNP is more toxic than SNAP, due to synergism of NO with cyanide released by SNP. SNAP elicits a Ca2+ influx in over half of the OLs within min. Further, OL death due to NO release from SNAP is Ca2+-dependent, because the Ca2+ chelator EGTA protects OLs from killing by SNAP, and also from killing by the NONOates NOC-9 and NOC-18, which spontaneously release NO. SNP does not elicit a Ca2+ influx, and EGTA is not protective. In comparison to the N20.1 OL cell line (Boullerne et al., [1999] J. Neurochem. 72:1050-1060), mature OLs are (1) more sensitive to SNAP, (2) much more resistant to SNP, (3) sensitive to cyanide, but not iron, and (4) exhibit a Ca2+ influx and EGTA protection in response to NO generated by SNAP.

Nitric oxide donors induce large-scale deletion mutations in human lymphoblastoid cells: implications for mutations in T-lymphocytes from arthritis patients

Rheumatoid arthritis (RA) is an inflammatory disease in which high levels of reactive nitrogen oxygen species (RNOS) may be present in the affected joints. RNOS are known to produce small-scale mutational events (transitions, transversions, small insertions, and small deletions) but the ability of these compounds to cause deletion of large segments of genomic DNA has not been previously determined. To address this question, a human lymphoblastoid cell line (WIL2-NS) was exposed to nitric oxide (NO)-donating drugs and hypoxanthine phosphoribosyltransferase (hprt)-negative clones were selected and analyzed by multiplex-PCR. Large-scale deletions accounted for 60-80% of hprt mutations arising in drug-treated cultures compared to 12% in untreated cultures (P-values of 0.006 and 0.0001, respectively, in two experiments). Deletion mutations in untreated cultures affected exon 9, whereas 75% of drug-induced deletion mutations affected exons 2, 3, and 9, and the remainder were very large, ranging from 26 to 1200 kbp. To compare this spectrum of NO-induced mutations in a lymphoblastoid line to that arising in vivo in arthritis patients, T-cells from RA patients, osteoarthritis (OA) patients, and controls were cloned and similarly analyzed. We previously showed that the overall frequency of Hprt mutant clones from patients is appreciably elevated compared to that of control subjects. Large-scale hprt deletions (0.5 to >26 kb) were detected in mutant T-cell clones from both RA and OA patients and also from control subjects. A total of 54 mutant clones from 16 RA patients and 19 mutant clones from 6 OA patients were studied. Of these, 6 clones (from 3 RA and 1 OA patient) had suffered large-scale deletions. A total of 9 control subjects were studied and 62 mutant clones were obtained. Of these, 19 had suffered large-scale deletions, arising in 7 of 9 control subjects. In conclusion, (1) RNOS are capable of inducing large-scale deletion mutations in a human lymphoblastoid cell line and (2) large-scale deletion mutations were found in 10-30% of T-cell clones from RA and OA patients and controls, which we hypothesize may be induced by RNOS.

Depletion of intracellular glutathione reduces mutations by nitric oxide-donating drugs

The Mutatect system is a mouse tumor line in which mutations at the hypoxanthine phosphoribosyltransferase (Hprt) locus can be readily detected both in vitro and in vivo. We have previously shown that the nitric oxide-generating drugs, glyceryl trinitrate (GTN) and sodium nitroprusside (SNP), can induce mutations that are readily detected in these cells. In the present report, we have tested the effect of glutathione depletion by buthionine sulfoximine (BSO) on cytotoxicity and mutagenicity by these two drugs. Exposure for 24 h to either drug (123 microM GTN; 500 microM SNP) induced mutations with relatively little cytotoxicity. Pretreatment with 50 microM BSO for 24 h, and then removal at the time of GTN or SNP addition, enhanced cytotoxicity to a modest extent. However, mutagenicity induced by both GTN and SNP was largely abolished. BSO did not affect nitrite accumulation in the medium over a 24-h period, indicating no inhibition of bioactivation of GTN or SNP. Maintaining BSO in the medium for 24 h prior and throughout the period of exposure to GTN or SNP produced a similar effect on mutations. N-Acetylcysteine and oxothiazolidine-4-carboxylate, drugs that are used to increase intracellular glutathione, also blocked mutations. We postulate that a product of the reaction between nitric oxide and intracellular glutathione, such as GSNO or some species derived from it, is promutagenic.

Study on DNA strand breaks induced by sodium nitroprusside, a nitric oxide donor, in vivo and in vitro

Nitric oxide (NO) as well as its donors has been shown to generate mutation and DNA damage in in vitro assays. The objective of this study was to identify that DNA single-strand breaks (SSBs) could be elicited by NO, not only in vitro but also in vivo. The alkaline single-cell gel electrophoresis (SCGE) was performed to examine the DNA damage in g12 cells and the cells isolated from the organs of mice exposed to sodium nitroprusside (SNP). A modified method, in which neither collagenase nor trypsin was necessary, was used to prepare the single-cell suspension isolated from organs of mice. Results showed that the exposure of g12 cells to 0.13-0.5 micromol/ml SNP with S9 for 1 h induced a concentration-dependent increase in DNA SSBs in g12 cells. The significant increase in DNA migration and comet frequency has appeared in the cells isolated from the spleen, thymus, and peritoneal macrophages of mice after injecting i.p. SNP in the dosage range of 0.67-6.0 mg/kg b.wt for 1 h. However, no obvious increase in DNA strand breaks was observed in the cells isolated from the liver, kidney, lung, brain and heart obtained from the same treated mice. These results suggested that DNA SSBs could be induced by NO in some cells both in vivo and in vitro. There were organ differences in sensitivity in the mice exposed to NO. Spleen, thymus, and macrophages might be the important targets of NO.

Neutrophils, nitric oxide synthase, and mutations in the mutatect murine tumor model

Mutatect MN-11 is a tumor line that can be grown subcutaneously in syngeneic C57BL/6 mice. The frequency of spontaneously arising mutants at the hypoxanthine phosphoribosyltransferase (Hprt) locus was observed to be elevated as a result of in vivo growth. The objective of the present study was to identify factors in the tumor microenvironment that might explain this increase in mutant frequency (MF). When tumors were examined histologically, neutrophils were found to be the predominant infiltrating cell type. Quantitative estimates of the number of neutrophils and MF of tumors in different animals revealed a statistically significant correlation (r = 0.63, P < 0.0001). Immunohistochemical analysis for inducible nitric oxide synthase (iNOS) demonstrated its presence, mainly in neutrophils. Biochemical analysis of tumor homogenates for nitric oxide synthase (NOS) activity indicated a statistically significant correlation with MF (r = 0.77, P < 0.0001). Nitrotyrosine was detected throughout the tumor immunohistochemically; both cytoplasmic and nuclear staining was seen. To increase the number of infiltrating neutrophils, tumors were injected with chemoattractant interleukin-8 and prostaglandin E2. This produced a statistically significant increase in neutrophil content (P = 0.005) and MF (P = 0.0002). As in control MN-11 tumors, neutrophil content and MF were strongly correlated (r = 0.63, P = 0. 003). Because neutrophils are a potential source of genotoxic reactive oxygen and/or nitrogen species, our results support the notion that these tumor-infiltrating cells may be mutagenic and contribute to the burden of genetic abnormalities associated with tumor progression.

Mutatect: a mouse tumour model for detecting radiation-induced mutations in vivo

A new mouse model (Mutatect) that permits detection of mutations at the hprt (hypoxanthine phosphoribosyltransferase) locus is described. It is highly sensitive to detection of mutants induced by clastogenic agents such as ionizing radiation. MN-11 cells are grown as a subcutaneous tumour in C57BL/6 mice for a period of 2 weeks, during which time they can be exposed to mutagenic treatments. Cells taken from the animal are cultured ex vivo and 6-thioguanine (6-TG)-resistant mutant clones can be readily identified and scored. This model system may have special utility for detecting multi-locus deletion events (chromosomal mutations) induced by high LET forms of radiation that might be encountered in space.

In vitro cytotoxicity of the nitric oxide donor, S-nitroso-N-acetyl-penicillamine, towards cells from human oral tissue

The cytotoxicity of the nitric oxide donor, S-nitroso-N-acetyl-penicillamine (SNAP), towards cultured human cells from oral tissue was evaluated. The toxicity of SNAP to Smulow-Glickman gingival epithelial cells was correlated with the liberation of nitric oxide, as N-acetyl-D,L-penicillamine, the SNAP metabolites, N-acetyl-D,L-penicillamine disulfide and nitrite, and preincubated (denitrosylated) SNAP did not affect viability. Comparing equimolar concentrations of various nitric oxide donors, cytotoxicity appeared to be inversely related to the relative stability (i.e., half-life) of the test compound; the sequence of cytotoxicity for a 4 hr exposure was S-nitrosoglutathione>>spermine NONOate> SNAP>DPTA NONOate>>DETA NONOate. Intracellular reduced glutathione (GSH) was lowered in S-G cells exposed to SNAP. Pretreatment of the cells with the GSH depleter, 1,3-bis-(chloroethyl)-1-nitrosourea (BCNU), enhanced the toxicity of SNAP Similar findings of enhanced sensitivity to SNAP were noted with gingival fibroblasts and periodontal ligament cells pretreated with BCNU. The toxicity of SNAP towards the gingival epithelial cells was decreased by cotreatment with the antioxidants, N-acetyl-L-cysteine, L-ascorbic acid, and (+)-catechin. Cells exposed to SNAP exhibited nuclear aberrations, including multilobed nuclei and multinucleation. SNAP-induced cell death was apparently by apoptosis, as noted by fluorescence microscopy and DNA agarose gel electrophoresis.

Synergism of nitric oxide and iron in killing the transformed murine oligodendrocyte cell line N20.1

Nitric oxide (NO) produced in inflammatory lesions may play a major role in the destruction of oligodendrocytes in multiple sclerosis and experimental allergic encephalomyelitis. The transformed murine oligodendroglial line N20.1 is much more resistant than primary oligodendrocytes to killing by the NO generator S-nitroso-N-acetyl-DL-penicillamine (SNAP). This observation prompted investigation of the mechanisms leading to cell death in the N20.1 cells and comparison of SNAP with another NO donor, sodium nitroprusside (SNP). We observed that N20.1 cells were 30 times more sensitive to SNP than to SNAP. The specific NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) protected against SNP only, not against SNAP. However, dithiothreitol protected against both SNAP and SNP, indicating that S-nitrosylation of cysteines plays a major role in the cytotoxicity of both NO donors. We did not observe any formation of peroxynitrite or increase of Ca2+ concentration with either SNAP or SNP, thus excluding their involvement in the mechanisms leading to N20.1 cell death. Based on two observations, (a) potentiation of the cytotoxic effect of SNP when coincubated with ferricyanide or ferrocyanide, but not sodium cyanide, and (b) protection by deferoxamine, an iron cyanide chelator, we conclude that the greater sensitivity of N20.1 cells to SNP compared with SNAP is due to synergism between NO released and the iron cyanide portion of SNP, with the cyanide accounting for very little of the cytotoxicity. Finally, SNP but not SNAP induces some apoptosis, as shown by DNA laddering and protection by a caspase-3 inhibitor. These results suggest that low levels of NO in combination with increased iron content lead to apoptotic cell death rather than the necrotic cell death seen with higher levels of NO generated by SNAP.