Induction of kinetochore positive and negative micronuclei in V79 cells by the alkylating agent diethylsulphate

Diethyl sulfate-induced cell cycle arrest and apoptosis in human bronchial epithelial 16HBE cells

In this study, we investigated the effects of diethyl sulfate (DES) on cell proliferation, cell cycle progression and apoptosis in human bronchial epithelial 16HBE cells. Cells were treated with various doses of DES (0, 0.5, 1.0, 2.0, 4.0 or 8.0mM) for 12, 24 or 36h. Cell proliferation and apoptosis were determined by MTT assay and flow cytometer, respectively. The results showed that DES inhibited cell proliferation in a dose- and time-dependent manner, and induced significant apoptosis in 16HBE cells. Apoptosis related proteins measurement results revealed that DES-induced apoptosis was concurrent with the increasing of Bax and cleavage fragment caspase-3 and the decreasing of Bcl-2 and full length procaspase-3. When cells were incubated with 2.0mM of DES for several time intervals, S and G2/M phase accumulation was observed. Further analysis indicated that both DES-induced G1/S transition acceleration and S arrest resulted in S phase accumulation, and that DES-induced G2/M arrest resulted in G2/M phase accumulation. Western blotting results demonstrated that after DES treatment p-chk1 (Ser345) and p-chk2 (Thr68) levels decreased in G1 cells, and increased in S and G2/M cells. In addition, the increasing of chk1 and chk2 were also induced by DES treatment. With the increase in the dose of DES, p53 levels first increased (0.5-4.0mM) and then decreased (8.0mM). Down-regulation of p53 by RNA interference increased 4.0mM of DES-induced apoptosis but did not affect 2.0mM DES-induced cell cycle arrest. In conclusion, DES inhibits 16HBE cells proliferation in a dose- and time-dependent behavior. Within the sublethal dose, DES induces S and G2/M arrest through activating DNA damage checkpoints. Within the lethal dose, DES induces apoptosis through evoking apoptosis programs. p53 might play an important role in the transition between evoking cell cycle arrest/pro-survival and apoptosis programs upon DES exposure.

Genotoxicity of 7H-dibenzo[c,g]carbazole and its tissue-specific derivatives in human hepatoma HepG2 cells is related to CYP1A1/1A2 expression

The goal of this study was to investigate the genotoxicity of 7H-dibenzo[c,g]carbazole (DBC), a ubiquitous environmental pollutant, and its methyl derivatives, 5,9-dimethylDBC (DiMeDBC), a strict hepatocarcinogen, and N-methylDBC (N-MeDBC), a specific sarcomagen in human hepatoma HepG2 cells, and to infer potential mechanisms underlying the biological activity of particular carcinogen. All dibenzocarbazoles, regardless the tissue specificity, induced significant DNA strand break levels and micronuclei in HepG2 cells; though a mitotic spindle dysfunction rather than a chromosome breakage was implicated in N-MeDBC-mediated micronucleus formation. While DBC and N-MeDBC produced stable DNA adducts followed with p53 protein phosphorylation at Ser-15, DiMeDBC failed. A significant increase in DNA strand breaks following incubation of exposed cells with a repair-specific endonuclease (Fpg protein) suggested that either oxidative DNA damage or unstable DNA-adducts might underlie DiMeDBC genotoxicity in human hepatoma cells. DiMeDBC and N-MeDBC increased substantially also the amount of CYP1A1/2 expression in HepG2 cells. Pretreatment of cells with substances affecting AhR-mediated CYP1A family of enzymes expression; however, diminished DiMeDBC and N-MeDBC genotoxicity. Our data clearly demonstrated differences in the mechanisms involved in the biological activity of DiMeDBC and N-MeDBC in human hepatoma cells; the genotoxicity of these DBC derivatives is closely related to CYP1A1/2 expression.

Micronucleus formation detected by live-cell imaging

Although micronuclei (MNi) have been extensively used to evaluate genotoxic effects and chromosome instability, the most basic issue regarding their formation was not completely addressed until recently, due to limitations of traditional experimental methods. The development of live-cell imaging, combined with genetically engineered chromosome labelling techniques makes it possible to investigate the origin of a micronucleus in a single cell in a real-time and high-throughput manner. Here, we review all the available studies on the origins of MNi in live cells and discuss novel findings based on this recently emerged methodology. Some unsolved questions on MNi formation and limitations of live-cell imaging in the investigation of MNi have also been discussed.

Factors Influencing the DNA Content of Radiation-induced Micronuclei

The distribution of the DNA content of radiation-induced micronuclei was analysed in several cell lines (Chinese hamster, Syrian hamster and mouse NIH-3T3 cells) by flow cytometry. Frequency and DNA content of micronuclei were measured simultaneously using fluorescence and forward scatter signals of micronuclei and nuclei in suspension stained with ethidium bromide. Computerized random breakage of chromosomes and random combination of fragments was performed to compare the measured micronucleus distributions in synchronized cells irradiated during G1-phase with calculated distributions. The measured DNA distribution of radiation-induced micronuclei was found to be influenced by several factors: (1) the DNA distribution and the centromeric index of the chromosomes in the various cell lines; (2) the cell cycle phase at time of micronucleus measurement due to DNA synthesis in micronuclei; (3) the presence of chromosome fragments in micronuclei; and (4) the presence of whole chromosomes in micronuclei. These factors were shown to be responsible for the previously found large radiation-induced micronuclei which could not be explained by the classic assumption only that radiation-induced micronuclei are mainly produced by single acentric fragments.

Multiple origins of spontaneously arising micronuclei in HeLa cells: direct evidence from long-term live cell imaging

Although micronuclei (MNi) are extensively used to evaluate genotoxic effects and chromosome instability, the most basic issue regarding their origins has not been completely addressed due to limitations of traditional methods. Recently, long-term live cell imaging was developed to monitor the dynamics of single cell in a real-time and high-throughput manner. In the present study, this state-of-the-art technique was employed to examine spontaneous micronucleus (MN) formation in untreated HeLa cells. We demonstrate that spontaneous MNi are derived from incorrectly aligned chromosomes in metaphase (displaced chromosomes, DCs), lagging chromosomes (LCs) and broken chromosome bridges (CBs) in later mitotic stages, but not nuclear buds in S phase. However, most of bipolar mitoses with DCs (91.29%), LCs (73.11%) and broken CBs (88.93%) did not give rise to MNi. Our data also show directly, for the first time, that MNi could originate spontaneously from (1) MNi already presented in the mother cells; (2) nuclear fragments that appeared during mitosis with CB; and (3) chromosomes being extruded into a minicell which fused with one of the daughter cells later. Quantitatively, most of MNi originated from LCs (63.66%), DCs (10.97%) and broken CBs (9.25%). Taken together, these direct evidences show that there are multiple origins for spontaneously arising MNi in HeLa cells and each mechanism contributes to overall MN formation to different extents.

Aneugenic potential of the nitrogen mustard analogues melphalan, chlorambucil and p-N,N-bis(2-chloroethyl)aminophenylacetic acid in cell cultures in vitro

Melphalan (MEL), chlorambucil (CAB) and p-N,N-bis(2-chloroethyl)aminophenylacetic acid (PHE) are nitrogen mustard analogues, which are clinically used as chemotherapeutic agents. They also exert carcinogenic activity. The aim of this study was to investigate the aneugenic potential of the above drugs and the possible mechanism responsible for this activity. The Cytokinesis Block Micronucleus (CBMN) assay in combination with fluorescence in situ hybridization (FISH) was used in human lymphocyte cultures to evaluate micronucleus (MN) frequency. Pancentromeric probe (alpha-satellite) was applied to identify chromosomes in micronuclei and an X-chromosome specific centromeric probe was used to asses micronucleation and non-disjunction of this chromosome in binucleated cells. The effect of the above compounds on the organization of mitotic apparatus, as a possible target of chemicals with aneugenic potential, was investigated in C(2)C(12) mouse cell line by double immunofluorescence of alpha- and gamma-tubulin. We found that the studied drugs increased MN frequency in a linear dose-dependent manner primarily by chromosome breakage and in a lesser extent by an aneugenic mechanism. Non-disjunction and micronucleation of X-chromosome were also induced. Abnormal metaphase cells were linearly increased with concentration and characterized by abnormal centrosome number. Interphase cells with micronuclei and abnormal centrosome number were also observed. Since nitrogen mustards are highly reactive agents, with low selectivity and form covalent bonds with different nucleophilic sites in proteins and nucleic acids, it is reasonable to consider that one possible pathway for nitrogen mustard analogues to exert their aneugenic activity is through reaction with nucleophilic moieties of proteins or genes that are involved in the duplication and/or separation of centrosomes, resulting in abnormal centrosome number. Based on our results the carcinogenicity of nitrogen mustard analogues studied may be attributed not only to their activity to trigger gene mutation and chromosome breakage, but also to their aneugenic potential. Further studies are warranted to clarify the above two hypotheses.

The use of the in vitro micronucleus assay to detect and assess the aneugenic activity of chemicals

The successful validation of the in vitro micronucleus assay by the SFTG now provides the opportunity for this highly cost effective assay to be used to screen chemicals for their ability to induce both structural (clastogenic) and numerical (aneugenic) chromosome changes using interphase cells. The use of interphase cells and a relatively simple experimental protocol provides the opportunity to greatly increase the statistical power of cytogenetic studies on chemical interactions. The application of molecular probes capable of detecting kinetochores and centromeres provides the opportunity to classify mechanisms of micronucleus induction into those which are primarily due to chromosome loss or breakage. When a predominant mechanism of micronucleus induction has been shown to be based upon chromosome loss then further investigation can involve the determination of the role of non-disjunction in the induction of aneuploidy. The binucleate cell modification of the in vitro micronucleus assay can be combined with the use of chromosome specific centromere probes to determine the segregation of individual chromosomes into daughter nuclei. The combination of these methods provides us with powerful tools for the investigation of mechanisms of genotoxicity particularly in the low dose regions.

Altered centrosomes in ataxia-telangiectasia cells and rapamycin-treated Chinese hamster cells

Rapamycin induces chromosome malsegregation in mammalian cell lines and yeast. Previous studies indicate that the function impaired in ataxia-telangiectasia (A-T) patients is necessary for both the growth inhibition and the chromosome malsegregation induced by rapamycin, and that treating the non-tumorigenic Chinese hamster cell line CHEF/18 with rapamycin results in supernumerary centrosomes and multipolar spindles. In this paper we report that lymphoblastoid cell lines established from A-T patients as well as hamster A-T-like cells are more resistant to rapamycin than the respective normal cell lines. Two cell lines derived from Nijmegen Breakage Syndrome (NBS) patients, who have clinical symptoms similar to those of A-T but a different molecular defect, were not resistant to rapamycin. Both A-T lymphoblastoid cells and A-T-like fibroblasts had giant centrosomes formed by more than two areas of gamma-tubulin-reacting material. Such giant centrosomes were also observed in CHEF/18 cells after prolonged treatment with rapamycin. Formation of giant centrosomes, possibly due to the coalescence of supernumerary centrosomes, was associated with increased aneuploidy in treated cells. Expression analysis of cell-cycle regulatory genes in rapamycin-treated human lymphoblastoid cells indicated that rapamycin decreased the expression of the tumor suppressor gene GADD45. The levels of RB, p21 and p53 mRNA were also decreased, although to a lesser extent. As rapamycin is often used as an immunosuppressant in pediatric transplant patients, these data indicate that caution should be taken, especially when the drug is given for prolonged periods of time.

Induction of micronuclei by 7H-dibenzo[c,g]carbazole and its tissue specific derivatives in Chinese hamster V79MZh1A1 cells

The clastogenicity/aneugenicity of N-heterocyclic polycyclic aromatic pollutant 7H-dibenzo[c,g]carbazole (DBC) and its two synthetic derivatives N-methyl DBC (MeDBC) and 5,9-dimethyl DBC (diMeDBC) was evaluated in the genetically engineered Chinese hamster V79 cell line V79MZh1A1 with stable expression of human cytochrome P4501A1 and in the parental V79MZ cell line without any cytochrome P450 activity. While none of the three carbazoles changed significantly the level of micronuclei in the parental V79MZ cells, a variable, but statistically significant rise of micronucleus frequencies was assessed in V79MZh1A1 cells. DBC induced dose-dependent increase in the number of micronuclei at harvest times of 24 and 48h and MeDBC at sampling time of 48h in V79MZh1A1 cells in comparison to untreated cells, however, no significant time-dependent increase in micronucleus frequencies was found. The use of the antikinetochore immunostaining revealed that DBC and MeDBC induced approximately equal levels of both kinetochore positive (C+) and kinetochore negative (C-) micronuclei. DiMeDBC, a strict hepatocarcinogen, did not manifest any effect on micronucleus induction in V79MZh1A1 cells. These studies suggest that genetically engineered Chinese hamster V79 cell lines expressing individual CYP cDNAs are a useful in vitro model for evaluation the role of particular cytochromes P450 in biotransformation of DBC and its tissue and organ specific derivatives.

Influence of nucleotide excision repair and of dose on the types of vermilion mutations induced by diethyl sulfate in postmeiotic male germ cells of Drosophila

The role of a defect for nucleotide excision repair (NER) in oocytes on the repair of DNA ethyl adducts induced by diethyl sulfate (DES) in male germ cells of Drosophila was analysed. Frequencies of mutations at multiple loci (recessive lethal mutations) and at the vermilion gene induced in NER+ conditions (cross NER+ x NER+) were compared with those fixed in a NER- background (NER- x NER+). The M(NER-)/M(NER+) mutability ratios for two DES concentrations, 10 mM and 15 mM, were 2.21 and 1.49, respectively, indicating that NER repairs part of the DES-induced damage. The majority of 28 fertile vermilion mutations produced by DES in NER- are transitions, both GC-AT (46.4%) and AT-GC (21.4%) transitions are found, the consequences of O6-ethylguanine and O4-ethylthymine, respectively. Transversions (21.5%), one +1 frameshift mutation (3.6%) and two deletions (7.1%) are most likely the result of N-alkylation damage. Furthermore, the DES-induced mutation spectra show interesting differences in relation to the exposure dose. All 10 mutants isolated in this and a previous [L.M. Sierra, A. Pastink, M.J.M. Nivard, E.W. Vogel, DNA base sequence changes induced by DES in postmeiotic male germ cells of Drosophila melanogaster, Mol. Gen. Genet. 237 (1993) 370-374] study from experiments with low DES-effectiveness are exclusively transitions, independent whether the females were of the NER+ or NER-genotype. This indicates that at lower DES effectiveness only O-alkylation damage is relevant, and that N-alkylation damage is repaired. In experiments revealing high DES-effectiveness, vermilion mutations representing N-alkylation damage reached 43% (9/21) with NER- and 26% (7/27) with NER+ females, suggesting (i) that NER becomes involved at high adduct levels because then the base excision repair (BER) may be saturated, and (ii) that this involvement of NER causes the relative decrease from 43% to 26% N-alkylation mediated sequence changes.

Antimutagenic effects of stobadine: review of results

The paper summarizes the results of our previously published studies testifying the hypothesis of the antimutagenic effect of stobadine (STB) in vivo and in vitro. The micronucleus test was used in in vivo experiments with ICR mice. Oral pretreatment with STB significantly decreased the mutagenic effect of cyclophosphamide (CP) in a concentration-dependent way. The protective effect of STB was confirmed in fetuses of CP-treated mice. STB pretreatment exerted also a radioprotective effect in Co60-irradiated mice. The ineffectiveness of STB posttreatment is indicative of its effect operative in the initiation of mutagenesis and of its radical-scavenging mechanism. The ability of STB to reduce N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)induced gene mutations and MNNG-induced calcinosis/Raynaud's phenomenon/esophageal dysmotility/sclerodactyly/telangiectasia variant of scleroderma (CREST)-positive and CREST-negative micronuclei in V79 cells was tested in in vitro experiments. We found that this drug reduced the level of both gene mutations and CREST-negative micronuclei mainly if given as pretreatment before exposure of cells to MNNG. We conclude that STB may have inhibited mutagenesis not only by scavenging reactive oxygen species, but also as a result of induction of metabolic enzymes, which reduced the level of DNA lesions.

Cytotoxic and genotoxic effect of inhibitor of vulcanisation N-cyclohexylthiophthalimide in a battery of in vitro assays

Mutagenicity of N-cyclohexylthiophthalimide (Duslin P) was tested first by the Ames test in the bacteria, Salmonella typhimurium. The negative results of the Ames test suggested that this compound does not induce mutations in the genome of S. typhimurium under the conditions used. To estimate the cytotoxicity of Duslin P to human cells, we measured cellular DNA and protein as well as cell proliferation, i.e., the mitotic index of treated and control cells. The genotoxic effects were assayed by two biochemical methods developed for detection of single-strand breaks of DNA in mammalian cells, i.e., by the alkaline single cell gel electrophoresis (comet assay) and by the DNA unwinding method, respectively. The DNA unwinding method showed that this compound did not induce DNA damage at concentrations < 7 micrograms/ml. Alkaline single cell gel electrophoresis revealed approximately double the level of DNA damage (in comparison to untreated control DNA) at a concentration of 2 micrograms/ml, which reduced proliferation to approximately 30%, and triple the level of DNA damage at higher concentrations (6 and 7 micrograms/ml), which inhibited completely both DNA synthesis and proteosynthesis. Cells with moderately damaged DNA were more common than cells with heavily damaged DNA. Parallel experiments with the strong mutagen and carcinogen MNNG showed that MNNG induced in cells a high level of DNA damage at concentrations which did not reduce the mitotic index or proteosynthesis, while DNA synthesis inhibited only partially. After treatment with MNNG, cells with heavily damaged DNA were more common than cells with moderately damaged DNA. Duslin P-treated VH10 cells were also tested cytogenetically, confirming that Duslin P induced neither chromosomal aberrations nor aneuploidy. We conclude that Duslin P has no mutagenic effect on bacteria, does not induce chromosomal aberrations and CREST positive or CREST negative micronuclei in human cells and induces only a small increase of DNA damage in human cells which is consistent with DNA fragmentation due to cell death.

Derivative chromosome 17 in a case of Burkitt lymphoma with 8;14 translocation

A complex chromosome rearrangement present in a B-cell line established from a patient with Burkitt lymphoma was studied by using fluorescence in situ hybridization (FISH) and immunocytochemistry techniques. The rearranged chromosome (der17) was apparently composed of 17q, of a partially deleted 17p, and of other material of chromosome 17p origin that was interspersed with regions without any clear banding pattern. der(17) contained a functional ch17 centromere and two additional centromeres of unknown origin that were inactive by all evidence. By FISH analysis with a TP53 probe, a signal could be demonstrated on the normal ch17, but not on the rearranged chromosome, a finding which indicates that 17p deletion caused a concurrent loss of one of the two TP53 alleles. The marker chromosome was previously observed in some of the malignant cells obtained from the patient's peripheral blood. These observations therefore indicate that cells with this specific rearrangement were generated in vivo and subsequently selected. This rearrangement is likely to have conferred a selective growth advantage to a subclone present in the original malignant cell population.

Fragile sites at the centromere of Chinese hamster chromosomes: a possible mechanism of chromosome loss

On the basis of our previous observations showing that fragile sites (FS) mapped essentially in the centromeric regions of Chinese hamster chromosomes, we consider the possibility that the presence of FS at the centromere might be a source of chromosome loss. In this model a centromeric FS causes a centromeric break giving rise to two chromosome arms which could be lost or maintained with different consequences on the ploidy of daughter cells. To test this hypothesis, Chinese hamster cells have been treated both with N-methyl-N-nitrosourea (MNU), a mutagenic agent which also induces aneuploidy, and vinblastin (VBL), a pure aneugen, used as a control compound, which is supposed not to interact with DNA. The results show that MNU induces the formation of translocated and/or truncated chromosomes, on the contrary VBL is not able to induce chromosome rearrangements. The sites most involved in MNU-induced breaks are the centromeric regions of chromosomes where FS are also present. These breaks cause essentially the loss of one chromosome arm, so that the resulting cells are numerically diploid but presenting partial monosomies. The implications of these results are discussed.

Cells from ERCC1-deficient mice show increased genome instability and a reduced frequency of S-phase-dependent illegitimate chromosome exchange but a normal frequency of homologous recombination

The ERCC1 protein is essential for nucleotide excision repair in mammalian cells and is also believed to be involved in mitotic recombination. ERCC1-deficient mice, with their extreme runting and polyploid hepatocyte nuclei, have a phenotype that is more reminiscent of a cell cycle arrest/premature ageing disorder than the classic DNA repair deficiency disease, xeroderma pigmentosum. To understand the role of ERCC1 and the link between ERCC1-deficiency and cell cycle arrest, we have studied primary and immortalised embryonic fibroblast cultures from ERCC1-deficient mice and a Chinese hamster ovary ERCC1 mutant cell line. Mutant cells from both species showed the expected nucleotide excision repair deficiency, but the mouse mutant was only moderately sensitive to mitomycin C, indicating that ERCC1 is not essential for the recombination-mediated repair of interstrand cross links in the mouse. Mutant cells from both species had a high mutation frequency and the level of genomic instability was elevated in ERCC1-deficient mouse cells, both in vivo and in vitro. There was no evidence for an homologous recombination deficit in ERCC1 mutant cells from either species. However, the frequency of S-phase-dependent illegitimate chromatid exchange, induced by ultra violet light, was dramatically reduced in both mutants. In rodent cells the G1 arrest induced by ultra violet light is less extensive than in human cells, with the result that replication proceeds on an incompletely repaired template. Illegitimate recombination, resulting in a high frequency of chromatid exchange, is a response adopted by rodent cells to prevent the accumulation of DNA double strand breaks adjacent to unrepaired lesion sites on replicating DNA and allow replication to proceed. Our results indicate an additional role for ERCC1 in this process and we propose the following model to explain the growth arrest and early senescence seen in ERCC1-deficient mice. In the absence of ERCC1, spontaneously occurring DNA lesions accumulate and the failure of the illegitimate recombination process leads to the accumulation of double strand breaks following replication. This triggers the p53 response and the G2 cell cycle arrest, mediated by increased expression of the cyclin-dependent kinase inhibitor p21(cip1/waf1). The increased levels of unrepaired lesions and double strand breaks lead to an increased mutation frequency and genome instability.

Induction of kinetochore positive and negative micronuclei in V79 cells by N-methyl-N'-nitro-N-nitrosoguanidine: the protective effect of the pyridoindole antioxidant stobadine

The induction of micronuclei by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and their reduction by the cardioprotective synthetic antioxidant, stobadine were studied in hamster V79 cells cultured in vitro. The micronuclei derived from acentric fragments or from whole chromosomes were evaluated with the help of an immunofluorescent staining using antikinetochore antibodies from the serum of scleroderma (CREST syndrome) patients. Our results showed that MNNG (0.5 microgram/ml) induced mainly kinetochore-negative micronuclei. At 6, 24 and 48 h after MNNG treatment, we measured a 2.7-, 4.3-, and 7.0-fold increase, respectively, of kinetochore-negative micronuclei over the controls. The increase of kinetochore-positive micronuclei was rather low and represented at 6, 24 and 48 h, respectively 0.9-, 1.8- and 2.6-fold increases over the controls. Stobadine decreased the level of kinetochore-negative micronuclei at 6, 24 and 48 h to approximately one-half; the frequency of kinetochore-positive micronuclei was reduced only at 6 h. We suppose that the antioxidant stobadine reduces the induction of micronuclei by MNNG by scavenging of MNNG-induced highly reactive OH radicals which cause chromosomal damage.

Aflatoxin exposure and cytogenetic alterations in individuals from the Gambia, West Africa

Aflatoxin-albumin adducts in peripheral blood provide a measure of exposure to aflatoxin over the previous 2-3 months. In the present study, the levels of these adducts were determined in a group of individuals from The Gambia, West Africa and were compared in a cross-sectional study to the levels of various cytogenetic alterations (chromosomal aberrations, micronuclei, sister chromatid exchanges) in the same individuals to test whether an increase in genetic damage is associated with an increased exposure in this population. Of 35 subjects tested for aflatoxin-albumin adducts, all but 3 were positive. There were no correlations between the adduct level and the number of cytogenetic abnormalities at the individual level. A comparison of the cytogenetic alterations was made between Gambian individuals and a group of 22 healthy people from Italy where aflatoxin exposure is expected to be low. The levels of structural chromosomal aberrations, sister chromatid exchanges and micronuclei were all higher in the former group. Overall, these data are indicative of a higher exposure to genotoxins in Gambian subjects, one of which are aflatoxins, but suggest that more specific genetic markers of aflatoxin exposure are required to further examine the link between aflatoxin exposure and genetic alterations.

Simplified and optimized kinetochore detection: cytogenetic marker for late-G2 cells

Cytogenetic detection of kinetochore proteins using the CREST antibody coupled with secondary antibodies labeled with different fluorescent probes has been optimized for several in vitro mammalian cell lines. This study investigated selected parameters including the influence of common fixatives (methanol, ethanol, methanol:acetic acid (3:1)), detergents (Triton-X100, Tween), fluorescent probes (CY3, BODIPY, FITC), washing protocols, and an antifading agent (paraphenylenediamine) on the detection of kinetochore proteins in control and X-ray (240 kVp)-irradiated cells. Utilizing an optimized fixation and staining protocol, a brilliant visualization of kinetochores in interphase cells was obtained in control as well as X-ray-irradiated interhase cells. Application of this improved kinetochore staining methodology readily permits discriminating cells containing either single or paired kinetochores, the latter of which are characteristic of late-G2 phase and prophase cells.

Induction of kinetochore-containing micronuclei by exogenous O6-methylguanine requires conversion of the methylated base to a nucleotide

It has been reported that exogenous alkylated purines, such as O6-methylguanine (O6meG), induce aneuploidy in mammalian cells. It is shown here that the aneugenic effect of O6meG, evidenced by its ability to induce micronuclei in rodent cells, is dependent on its conversion to O6-methyl-guanosine-5'-monophosphate (O6me-5'-GMP) by hypoxanthine-guanine phosphoribosyl transferase (HPRT). This conclusion, in contrast with previous in vitro data showing that O6meG does not seem to be a substrate for HPRT, was based on the following observations: 1) O6meG did not induce micronuclei in HPRT-deficient Chinese hamster cells, but did induce micronuclei in HPRT-proficient cells, and in mouse cells partially or totally deficient in adenine phosphoribosyl transferase; 2) O6meG was not metabolized in HPRT-deficient cells, while in wild-type cells a number of metabolites were detected by high performance liquid chromatography (HPLC) analysis of cold acid extracts, one of them coeluting with O6me-5'-GMP used as a marker; 3) when de novo synthesis of purine nucleotides was inhibited by aminopterin, O6meG sustained the growth of HPRT-proficient, but not of HPRT-deficient, cells; and 4) when HPRT-deficient cells were treated with liposomes charged with O6me-5'-GMP, induction of micronuclei was shown. The finding that methylated guanine exerts its aneugenic action through methylated nucleotide(s) provides an important, though indirect, support to the hypothesis that alkylating agents may induce aneuploidy via nucleotide pool alkylation.

Genotoxic effects of the carbamate insecticide methomyl. I. In vitro studies with pure compound and the technical formulation "Lannate 25"

The carbamate insecticide methomyl and the methomyl-containing technical formulation "Lannate 25" were tested on whole blood human lymphocyte cultures. Both products induced dose-dependent increases in chromosome aberrations and micronuclei. Lannate 25 induced DNA damage as measured by the alkaline elution assay and hydroxylation of guanine at the C8 position. Sister chromatid exchanges were not increased significantly with either product. Overall, the technical formulation was more active than the pure compound, when compared at similar concentrations of active principle. Moreover, a different ratio of CREST-positive/CREST-negative micronuclei was observed with the two products, pure methomyl being relatively more active than Lannate 25 in the induction of CREST-positive micronuclei. On the basis of these results, previous evaluations of methomyl as a nongenotoxic compound should be reconsidered.

Further evaluation of a modified micronucleus assay with V79 cells for detection of aneugenic effects

In an earlier publication, we reported on the development of a modified micronucleus assay with V79 cells enabling preferential detection of aneugen-induced micronuclei (Seelbach et al., 1993). Here we present a further evaluation of the modified micronucleus assay based on the investigation of seven further suspected aneugens. Five compounds gave positive results: cadmium chloride, chloral hydrate, hydroquinone, thimerosal and vinblastine. Econazole and pyrimethamine were negative. Up to now, our experience has shown that data produced by the modified V79/micronucleus assay are quite reliable: the variation of spontaneous micronucleus frequencies was low (0.8-1.7%) and the reproducibility of the data was good.

A comparative study of the use of primary Chinese hamster liver cultures and genetically engineered immortal V79 Chinese hamster cell lines expressing rat liver CYP1A1, 1A2 and 2B1 cDNAs in micronucleus assays

Liver microsome preparations (S9 mix) have been extensively used for in vitro genotoxicity studies to provide the capacity for the activation of indirect genotoxins. However, the use of S9 preparations with mammalian cell cultures has raised considerable toxicity problems which limit their use to exposure times which are only a small fraction of the cell cycle. In addition, false negative results may be obtained if reactive metabolites are unable to penetrate the cell membrane or have short half-lives. The generation and detection of a promutagen within a single cell would therefore be advantageous. To this end, we have studied the bioactivation of a panel of promutagens (benzo[a]pyrene, cyclophosphamide, 2-aminoanthracene and sterigmatocystin) in low passage Chinese hamster fibroblasts of hepatic origin (LiC2 cells) and in a series of V79 Chinese hamster cell lines genetically engineered to express rat liver cytochrome P450 cDNAs. These include strains XEM2 (expresses CYP1A1), SD1 (CYP2B1) and strains XEMd-MZ and XEMd-NH which express CYP1A2. The end point selected for study was the induction of micronuclei. The protocol incorporated a cytochalasin B-induced cytokinesis block and the enumeration of micronuclei in the resulting binucleate cells which have undergone one nuclear division following the induction of chromosome damage. Micronuclei containing whole chromosomes and chromosome fragments were distinguished by the use of CREST antibody specific for kinetochore protein as a measure for the presence of centromeres. Micronuclei were induced by the test agents in low passage liver fibroblasts and in immortal V79 cultures only in the presence of Aroclor-induced S9 preparations. The data obtained from micronucleus assays of the genetically engineered V79 cell lines demonstrated the utility of each strain for the optimal detection and quantification of the activity of the individual test compounds. Kinetochore antibody demonstrated differences in the kinetics of induction of micronuclei containing chromosome fragments and whole chromosomes with chemicals such as benzo[a]pyrene. As part of this cytogenetic study, we also conducted karyotypic analyses and spindle fidelity assays of the V79 cell lines to investigate the presence of chromosomal instabilities which may arise as a consequence of the genetic engineering procedure. Such studies represent an important quality control step in the validation of the suitability of each cell line prior to their use in genotoxicity studies.

The cytochalasin-B micronucleus/kinetochore assay in vitro: studies with 10 suspected aneugens

An in vitro micronucleus assay in low passage Chinese hamster Luc2 cells capable of detecting numerical and structural chromosome changes was developed. Chromosome loss was inferred by indirect visualisation of human CREST antikinetochore antibodies bound to centromeres in chemically-induced micronuclei of cytochalasin-B arrested binucleated cells. The assay was used to evaluate 10 chemicals which had been selected for their known or suspected effects upon various components of the cell-division apparatus. These chemicals were colchicine (COL), vinblastine (VBL), thiabendazole (TBZ), chloral hydrate (CH), thimerosal (TM), diazepam (DZ), pyrimethamine (PYR), hydroquinone (HQ), cadmium chloride (CdCl2) and econazole nitrate (EZ). Mitomycin-C (MMC) was used as a positive control for the induction of micronuclei. 8 of the core chemicals induced micronuclei in Chinese hamster Luc2 cells. 4 of the chemicals (COL, VBL, TBZ, CH) increased levels of micronuclei which were positive for kinetochore antibody labelling and hence chromosome loss. 3 of the chemicals (DZ, PYR, HQ) and the positive control (MMC) increased the levels of Mn which were negative for kinetochore antibody labelling. The results with TM were equivocal and EN was negative. The results of these studies suggest that the cytochalasin-B Mn/k assay is a cost-effective, simple and rapid alternative to classical cytogenetic assays for the detection of chemically induced aneuploidy.

Chromosomes bearing amplified genes are a preferential target of chemicals inducing chromosome breakage and aneuploidy

Micronuclei were induced in V79 Chinese hamster cells and in PALA L and MTX M, two derivative cell lines harboring amplified genes, with 1,3-bis(2-chloroethyl)nitrosourea (BCNU) and vinblastine. Spontaneous and induced micronuclei were analyzed for the presence of centromeres by immunofluorescent CREST staining. Micronuclei formed in PALA L cells were also analyzed for the presence of amplified DNA by in situ hybridization with a CAD gene probe. Both cell lines containing amplified genes showed increased micronucleus induction by BCNU and vinblastine. The marker chromosome of PALA L cells was found to be a preferential target for both the clastogenic and the aneugenic action of the two chemicals. DNA amplification seems therefore to be a destabilizing factor of chromosomal structural integrity and mitotic segregation.

Genotoxicity analysis of N,N-dimethylaniline and N,N-dimethyl-p-toluidine

N,N-Dimethylaniline (DMA, CAS No. 121-69-7) and N,N-dimethyl-p-toluidine (DMPT, CAS No. 99-97-8) belong to the N-dialkylaminoaromatics, a chemical class structurally alerting to DNA reactivity. Their applications may be industrial (dye and pesticide intermediates, polymerizing agents) and surgical (polymerization accelerators for the manufacture of bone cements and prosthetic devices), thus implying heterogeneous types of human exposure. Findings of carcinogenicity in rodents and some nonexhaustive genotoxicity data are available for DMA, but to our knowledge no information is available on DMPT concerning either carcinogenicity or any kind of genetic toxicity. To investigate their mechanism of action and mutagenic/carcinogenic potential, DMA and DMPT were analyzed for complementary genotoxicity endpoints, namely, gene mutation in Salmonella (Ames test), structural and numerical chromosome aberrations in hamster V79 cells (micronucleus test, matched with an immunofluorescent staining for kinetochore proteins), and in vivo DNA damage in mouse and rat liver (alkaline DNA elution test). The results essentially indicate that both chemicals are chromosome damaging agents. Indeed, at the maximum nontoxic doses, they proved nonmutagenic in Salmonella (although their toxicity did not allow concentrations > 70 micrograms/plate to be tested) and weakly positive in inducing DNA damage (increases in DNA elution rates at most approximately 2.4 times control value). Conversely, they proved clearly positive in inducing numerical chromosome alterations, with dose-dependent increases up to more than five times the control value for DMPT. At the highest dose tested, both chemicals also showed a significant clastogenic effect.

Genotoxicity of N-acryloyl-N'-phenylpiperazine, a redox activator for acrylic resin polymerization

N-Acryloyl-N'-phenylpiperazine is a promoter of redox reactions synthesized recently, and proposed as an activator for the polymerization of acrylic resins for biomedical use. The chemical was analyzed for different genotoxicity endpoints, to obtain both information on its possible mutagenic/carcinogenic potential and a model analysis of a tertiary arylamine, which belongs to a class of chemicals commonly used as polymerization accelerators in the biomaterial field. The genotoxicity endpoints considered were: gene mutation in the Salmonella test; structural and numerical chromosome alterations in Chinese hamster V79 cells, evaluated by the micronucleus test together with an immunofluorescent staining specific for kinetochore proteins; in vitro and in vivo DNA damage, evaluated in V79 cells and in mouse liver by the alkaline DNA elution technique. On the whole, the results indicate that N-acryloyl-N'-phenylpiperazine is to be regarded not so much as a DNA-damaging agent, but as a genomic mutagen. Indeed, it was not mutagenic in Salmonella (though its toxicity did not allow testing concentrations over 70 micrograms/plate), and it was weakly positive in inducing chromosomal fragmentation in vitro (one positive, not dose-related, result out of five different doses tested) and in vivo DNA damage (increases in DNA elution rate never doubling control values). The chemical was, however, clearly positive (with dose-dependent effects up to about 25 times the control value) in causing numerical chromosome alterations, at the maximal non-toxic doses.

Multiparametric flow cytometric analysis of radiation-induced micronuclei in mammalian cell cultures

A new flow cytometric method is presented that quantifies the frequency of radiation-induced micronuclei in mammalian cell cultures with high precision. After preparing a suspension of main nuclei and micronuclei stained with ethidium bromide and Hoechst 33258, both types of particles are measured simultaneously in a flow cytometer using forward light scatter and three fluorescence emission intensities excited by UV, 488 nm, and by energy transfer from Hoechst 33258 to ethidium bromide. Nonspecific debris overlapping the micronucleus distribution especially in the low fluorescence intensity region was discriminated from micronuclei by calculating ratios of the different fluorescences. The frequencies of radiation-induced micronuclei measured with this new technique agreed well with results obtained by conventional microscopy. The lower limit of the DNA content of micronuclei identified by this technique was found to be about 0.5%-0.75% of the DNA content of G1-phase nuclei. Dose effect curves and the time-dependent induction of micronuclei were measured for two different mouse cell lines.

Identification of kinetochores and DNA synthesis in micronuclei induced by mitomycin C and colchicine in chinese hamster ovary cells

The presence of kinetochore and DNA synthesis in micronuclei (MN) induced in Chinese hamster ovary (CHO) cells by clastogenic and aneuploidogenic substances such as mitomycin C (MMC) and colchicine was determined by immunofluorescence technique using CREST antikinetochore antibodies and anti-bromodeoxyuridine (BrdUrd) antibodies. A cytofluorimetric analysis was also performed. Colchicine significantly increased micronucleated cells at least up to 96 h from the end of treatment. As expected, among colchicine-induced micronucleated cells the majority contained at least one CREST + MN. MMC induced a significant increase in micronucleated cells up to 120 h from the end of treatment and the great majority of MN lacked kinetochore fluorescence, indicating that MMC-induced MN were derived from acentric fragments. However, colchicine and MMC at 48 and 72 h from the end of treatment, induced a significant increase of CREST- and CREST + MN, respectively, suggesting an induction of clastogenicity by colchicine and aneuploidy by MMC. The clastogenic effect of colchicine after 48 h was also confirmed by the presence of chromatid fragments in metaphase cells. A cytofluorimetric analysis indicated that, as expected, colchicine and MMC interfere with the G2/M and S phases, respectively; however, a slight interference of colchicine with the S phase was also observed. DNA synthesis was present in MN and it was in most cases synchronous with synthesis in the main nucleus. The frequency of cells with MN in S phase observed in untreated or MMC-treated cells is in agreement with the proportion of cells without MN showing DNA synthesis. On the contrary, the frequency of cells with MN in S phase observed in colchicine-treated cells was significantly lower than that observed in control and MMC-treated cells.

Dislocation of chromatin elements in prophase induced by diethylstilbestrol: a novel mechanism by which micronuclei can arise

The in vitro micronucleus test with Syrian hamster embryo (SHE) cells assays the induction of micronuclei by chemical agents. Both chromosome fragments and lagging chromosomes can give rise to micronuclei. Nevertheless, only limited information is available on the ultrastructure of micronuclei and the mechanisms of their formation. Diethylstilbestrol (DES), a non-mutagenic carcinogen, as well as its analogue 3.3'-DES induce micronuclei in SHE cells. A comparison of the dose response of DES-induced micronucleus formation with the previously published ones for aneuploidy and transformation shows that all 3 run in parallel. Thus, a functional relationship between these endpoints, in the SHE system, may be implied. The present study is designed to address the formation of micronuclei using supravital UV microscopy, to test for the presence of defined chromosome domains within micronuclei using immunocytochemistry, and to define aspects of their ultrastructure by electron microscopy. Supravital UV microscopy showed that 3.3'-DES induces displacement of chromosomes/chromatids during prophase/anaphase and formation of micronuclei during cytokinesis. Immunocytochemistry revealed that micronuclei contain, at high frequencies, CREST antibody-reactive kinetochores, indicating the presence of whole chromosomes or centric fragments in these structures. Moreover, transmission electron microscopy showed that micronuclei exhibit ultrastructural details typical of interphase nuclei. Specifically, micronuclei exhibited morphological evidence of a nuclear lamina and segregation of karyoplasm into euchromatic and heterochromatic regions. All micronuclei examined were enclosed by a nuclear envelope of normal morphology and showed nuclear pore complexes. Together the findings provide evidence that DES interferes with the mitotic apparatus as early as prophase, resulting in the formation of micronuclei and, as a consequence, in the loss of chromatids or chromosomes.