Measurement of mutagenesis in mammalian cells

Alkylating agents are possible inducers of glioblastoma and other brain tumors

Epidemiological evidence of an association between exposure to chemical carcinogens and an increased risk for development of glioblastoma (GBM) is limited to weak statistical associations in cohorts of firefighters, farmers, residents exposed to air pollution, and soldiers exposed to toxic chemicals (e.g., military burn pits, oil-well fire smoke). A history of ionizing radiation therapy to the head or neck is associated with an increased risk of GBM. Ionizing radiation induces point mutations, frameshift mutations, double-strand breaks, and chromosomal insertions or deletions. Mutational profiles associated with chemical exposures overlap with the broad mutational patterns seen with ionizing radiation. Data on 16 agents (15 chemicals and radio frequency radiation) that induced tumors in the rodent brain were extracted from 602 Technical Reports on 2-years cancer bioassays found in the National Toxicology Program database. Ten of the 15 chemical agents that induce brain tumors are alkylating agents. Three of the 15 chemical agents have idiosyncratic structures and might be alkylating agents. Only two of the 15 chemical agents are definitively not alkylating agents. The rat model is thought to be of possible relevance to humans suggesting that exposure to alkylating chemicals should be considered in epidemiology studies on GBM and other brain tumors.

Risk factors for glioblastoma are shared by other brain tumor types

The reported risk factors for glioblastoma (GBM), i.e., ionizing radiation, Li-Fraumeni syndrome, Neurofibromatosis I, and Turcot syndrome, also increase the risk of other brain tumor types. Risk factors for human GBM are associated with different oncogenic mutation profiles. Pedigreed domestic dogs with a shorter nose and flatter face (brachycephalic dogs) display relatively high rates of glioma formation. The genetic profiles of canine gliomas are also idiosyncratic. The association of putatively different mutational patterns in humans and canines with GBM suggests that different oncogenic pathways can result in GBM formation. Strong epidemiological evidence for an association between exposure to chemical carcinogens and an increased risk for development of GBM is currently lacking. Ionizing radiation induces point mutations, frameshift mutations, double-strand breaks, and chromosomal insertions or deletions. Mutational profiles associated with chemical exposures overlap with the broad mutational patterns seen with ionizing radiation. Weak statistical associations between chemical exposures and GBM reported in epidemiology studies are biologically plausible. Molecular approaches comparing reproducible patterns seen in spontaneous GBM with analogous patterns found in GBMs resected from patients with known significant exposures to potentially carcinogenic chemicals can address difficulties presented by traditional exposure assessment.

Destabilizing Effects of Ionizing Radiation on Chromosomes: Sizing up the Damage

For long-term survival and evolution, all organisms have depended on a delicate balance between processes involved in maintaining stability of their genomes and opposing processes that lead toward destabilization. At the level of mammalian somatic cells in renewal tissues, events or conditions that can tip this balance toward instability have attracted special interest in connection with carcinogenesis. Mutations affecting DNA (and its subsequent repair) would, of course, be a major consideration here. These may occur spontaneously through endogenous cellular processes or as a result of exposure to mutagenic environmental agents. It is in this context that we discuss the rather unique destabilizing effects of ionizing radiation (IR) in terms of its ability to cause large-scale structural rearrangements to the genome. We present arguments supporting the conclusion that these and other important effects of IR originate largely from microscopically visible chromosome aberrations.

Aging-independent and size-dependent genotoxic response induced by titanium dioxide nanoparticles in mammalian cells

Titanium dioxide nanoparticles (TiO₂ NPs) are subjected to various transformation processes (chemical, physical and biological processes) in the environment, potentially affecting their bioavailability and toxic properties. However, the size variation of TiO₂ NPs during aging process and subsequent effects in mammalian cells are largely unknown. The aim of this study was to illustrate the adverse effects of TiO₂ NPs in different sizes (5, 15 and <100 nm) during aging process on human-hamster hybrid (A_L) cells. There was an aging-time dependent enhancement of average hydrodynamic size in TiO₂ NPs stock suspensions. The cytotoxicity of fresh TiO₂ NPs increased in a size-dependent manner; in contrast, their genotoxicity decreased with the increasing sizes of NPs. No significant toxicity difference was observed in cells exposed to either fresh or 60 day-aged TiO₂ NPs. Both Fresh and aged TiO₂ NPs efficiently induced mitochondrial dysfunction and activated Caspase-3/7 in a size-dependent manner. Using mitochondrial-DNA deficient (ρ⁰) A_L cells, we further discovered that mitochondrial dysfunction made significant contribution to the size-dependent toxicity induced by TiO₂ NPs during the aging process. Taken together, our data indicated that TiO₂ NPs could significantly induced the cytotoxicity and genotoxicity in an aging time-independent and size-dependent manner, which were triggered by mitochondrial dysfunction. Our study suggested the necessity to include size as an additional parameter for the cautious monitoring of TiO₂ NPs disposal before entering the environment.

Induction of somatic mutations by low-dose X-rays: the challenge in recognizing radiation-induced events

It is difficult to distinguish radiation-induced events from spontaneous events during induction of stochastic effects, especially in the case of low-dose or low-dose-rate exposures. By using a hypersensitive system for detecting somatic mutations at the HPRT1 locus, we investigated the frequency and spectrum of mutations induced by low-dose X-rays. The mutant frequencies induced by doses of >0.15 Gy were statistically significant when compared with the spontaneous frequency, and a clear dose dependency was also observed for mutant frequencies at doses of >0.15 Gy. In contrast, mutant frequencies at doses of <0.1 Gy occurred at non-significant levels. The mutation spectrum in HPRT-deficient mutants revealed that the type of mutations induced by low-dose exposures was similar to that seen in spontaneous mutants. An apparent change in mutation type was observed for mutants induced by doses of >0.2 Gy. Our observations suggest that there could be a critical dose for mutation induction at between 0.1 Gy and 0.2 Gy, where mutagenic events are induced by multiple DNA double-strand breaks (DSBs). These observations also suggest that low-dose radiation delivered at doses of <0.1 Gy may not result in DSB-induced mutations but may enhance spontaneous mutagenesis events.

Feline vaccine-associated sarcomagenesis: Is there an inflammation-independent role for aluminium?

Aluminium has been found in feline vaccine-associated sarcomas. In this study, we investigated the potential for aluminium to contribute directly to tumourigenesis. Our results indicated that an aluminium hydroxide adjuvant preparation was cytotoxic and mutagenic in human-Chinese hamster ovary (CHO) hybrid cells in vitro. Moreover, CHO cells deficient in DNA double strand break (DSB), but not single-strand break (SSB), repair, were particularly sensitive to aluminium exposure compared with repair proficient cells, suggesting that aluminium is associated with DSBs. In contrast to CHO cells, primary feline skin fibroblasts were resistant to the cytotoxic effects of aluminium compounds and exposure to an aluminium chloride salt promoted cell growth and cell cycle progression at concentrations much less than those measured in particular feline rabies vaccines. These findings suggest that aluminium exposure may contribute, theoretically, to both initiation and promotion of tumours in the absence of an inflammatory response.

The role of protein kinase C alpha translocation in radiation-induced bystander effect

Ionizing radiation is a well known human carcinogen. Evidence accumulated over the past decade suggested that extranuclear/extracellular targets and events may also play a critical role in modulating biological responses to ionizing radiation. However, the underlying mechanism(s) of radiation-induced bystander effect is still unclear. In the current study, A_L cells were irradiated with alpha particles and responses of bystander cells were investigated. We found out that in bystander A_L cells, protein kinase C alpha (PKCα) translocated from cytosol to membrane fraction. Pre-treatment of cells with PKC translocation inhibitor chelerythrine chloride suppressed the induced extracellular signal-regulated kinases (ERK) activity and the increased cyclooxygenase 2 (COX-2) expression as well as the mutagenic effect in bystander cells. Furthermore, tumor necrosis factor alpha (TNFα) was elevated in directly irradiated but not bystander cells; while TNFα receptor 1 (TNFR1) increased in the membrane fraction of bystander cells. Further analysis revealed that PKC activation caused accelerated internalization and recycling of TNFR1. Our data suggested that PKCα translocation may occur as an early event in radiation-induced bystander responses and mediate TNFα-induced signaling pathways that lead to the activation of ERK and up-regulation of COX-2.

Accelerated (48)Ti Ions Induce Autosomal Mutations in Mouse Kidney Epithelium at Low Dose and Fluence

Exposure to high-energy charged particles (HZE ions) at low fluence could significantly affect astronaut health after prolonged missions in deep space by inducing mutations and related cancers. We tested the hypothesis that the mutagenic effects of HZE ions could be detected at low fluence in a mouse model that detects autosomal mutations in vivo. Aprt heterozygous mice were exposed to 0.2, 0.4 and 1.4 Gy of densely ionizing (48)Ti ions (1 GeV/amu, LET = 107 keV/μm). We observed a dose-dependent increase in the Aprt mutant fraction in kidney epithelium at the two lowest doses (an average of 1 or 2 particles/cell nucleus) that plateaued at the highest dose (7 particles/cell nucleus). Mutant cells were expanded to determine mutation spectra and translocations affecting chromosome 8, which encodes Aprt. A PCR-based analysis for loss of heterozygosity (LOH) events on chromosome 8 demonstrated a significant shift in the mutational spectrum from Ti ion exposure, even at low fluence, by revealing "radiation signature" mutations in mutant cells from exposed mice. Likewise, a cytogenetic assay for nonreciprocal chromosome 8 translocations showed an effect of exposure. A genome-wide LOH assay for events affecting nonselected chromosomes also showed an effect of exposure even for the lowest dose tested. Considered in their entirety, these results show that accelerated (48)Ti ions induce large mutations affecting one or more chromosomes at low dose and fluence.

Mutagenicity of PFOA in mammalian cells: role of mitochondria-dependent reactive oxygen species

Mutagenicity is often a prerequisite to the development of malignancy. Evidences have shown that exposure to perfluorooctanoic acid (PFOA) results in various cancer inductions. However, whether any mutagenic base exists is still puzzling. In the present study, we exposed exponentially growing AL cells to PFOA and assayed the cells for survival, mutation induction, and caspase-3/7, -9 activities. Mitochondrial-DNA deficient human-hamster hybrid (ρ(0) AL) cells and reactive oxygen species (ROS) inhibitor were used to elucidate the possible mechanism. Our results showed that treatment of AL cells with PFOA for 16 days induced significant mutagenic effects together with the increment of ROS, superoxide anions (O2(.-)), and nitrogen oxide (NO) levels, while treatment of ρ(0) AL cells did not have much change. Concurrent treatment of AL cells with ROS inhibitor significantly decreased the mutagenic potential of PFOA. In addition, caspase activities in AL cells were increased by PFOA exposure and suppressed by ROS/RNS (reactive oxygen/nitrogen species) inhibitors. Our results suggest that exposure to PFOA lead to mutagenicity induction in AL cells, and mitochondria-dependent ROS plays an important role in this process. This provides a direct base for PFOA mediated cancer induction.

Mechanism of genotoxicity induced by targeted cytoplasmic irradiation

Background:

Direct damage to DNA is generally accepted as the main initiator of mutation and cancer induced by environmental carcinogens or ionising radiation. However, there is accumulating evidence suggesting that extracellular/extranuclear targets may also have a key role in mediating the genotoxic effects of ionising radiation. As the possibility of a particle traversal through the cytoplasm is much higher than through the nuclei in environmental radiation exposure, the contribution to genotoxic damage from cytoplasmic irradiation should not be ignored in radiation risk estimation. Although targeted cytoplasmic irradiation has been shown to induce mutations in mammalian cells, the precise mechanism(s) underlying the mutagenic process is largely unknown.

Methods:

A microbeam that can target the cytoplasm of cells with high precision was used to study mechanisms involved in mediating the genotoxic effects in irradiated human–hamster hybrid (A_L) cells.

Results:

Targeted cytoplasmic irradiation induces oxidative DNA damages and reactive nitrogen species (RNS) in A_L cells. Lipid peroxidation, as determined by the induction of 4-hydroxynonenal was enhanced in irradiated cells, which could be suppressed by butylated hydroxyl toluene treatment. Moreover, cytoplasmic irradiation of A_L cells increased expression of cyclooxygenase-2 (COX-2) and activation of extracellular signal-related kinase (ERK) pathway.

Conclusion:

We herein proposed a possible signalling pathway involving reactive oxygen/nitrogen species and COX-2 in the cytoplasmic irradiation-induced genotoxicity effect.

Hypoxia/reoxygenation-induced mutations in mammalian cells detected by the flow cytometry mutation assay and characterized by mutant spectrum

Under hypoxic conditions, cells are more resistant to cell killing by ionizing radiation by a factor of 2.5 to 3, potentially compromising the efficacy of radiotherapy. It has been shown recently that hypoxic conditions alone are sufficient to generate mutations in vitro and in vivo, likely due to the creation of reactive oxygen species (ROS) and a decrease in mismatch and homologous recombination DNA repair activity. These factors are known precursors to the onset of genetic instability and poor prognosis. We have previously characterized the flow cytometry mutation assay and its sensitivity to detect significant mutant fractions induced by genotoxic agents that are not detected by other mammalian assays. Here we measure the mutant fraction induced by hypoxia. CHO A(L) cells cultured at <0.1% O(2) for 24 h generated a significant mutant fraction of 120 x 10(-5) and had growth kinetics and survival characteristics similar to those obtained with other mutagens. We investigated the role of ROS by treating cells with the radical scavenger DMSO, which significantly reduced hypoxia toxicity and mutagenesis. Single cells were sorted from the mutant population, and the resulting clonal populations were stained for five antigens encoded by genes found along chromosome 11 to generate mutant spectra. The mutations were primarily large deletions, similar to those in background mutants, but the frequency was higher. We have demonstrated that hypoxic conditions alone are sufficient to generate mutations in mammalian cells in culture and that the spectrum of mutations is similar to background mutations.

Cytotoxicity and genotoxicity reporter systems based on the use of mammalian cells

With the dramatic increase in the number of new agents arising from the chemical, pharmaceutical, and agricultural industries, there is an urgent need to develop assays for rapid evaluation of potential risks to man and environment. The panel of conventional tests used for cytotoxicity and genotoxicity and the strategies to progress from small scale assays to high content screening in toxicology are discussed. The properties of components necessary as sensors and reporters for new reporter assays, and the application of genetic strategies to design assays are reviewed. The concept of cellular reporters is based on the use of promoters of chemical stress-regulated genes ligated to a suitable luminescent or fluorescent reporter gene. Current reporter assays designed from constructs transferred into suitable cell lines are presented.

Kinetics of CHO A L mutant expression after treatment with gamma radiation, EMS, and asbestos

The flow cytometry mutation assay (FCMA) uses hybrid CHO A(L) cells to measure mutations of the cd59 gene located on human chromosome 11 by the absence of fluorochrome-conjugated antibody binding to the CD59 surface antigen. Mutant expression peaks between 6 and 12 days, then decreases to a stable plateau, instead of a constant mutant fraction obtained by clonogenic assays. To evaluate this variable mutant expression time, cells were treated with radiation, EMS or asbestos and cell proliferation and survival were measured at times leading up to peak mutant expression. Potential doubling time (T(pot)) values increased by at least 75% for each agent by 3 h after treatment but returned to control levels after only 3 days. Survival returned to 90% of control within a week, close to the peak expression day for all three agents. The survival of CD59(-) cells sorted on the peak day of expression was roughly half that of CD59(+) cells. Cloned EMS-treated CD59(-) cells had a doubling time of 16.7 vs. 14.1 h for CD59(+) cells. Triple mutants (CD59(-)/CD44(-)/CD90(-)) were preferentially lost from the population over time, while the proportion of CD59(-)/CD90(-) mutants increased. In conclusion, the peak day of mutant expression occurs only when cells recover from the toxic effects of the mutagen. A fraction of cells originally quantified as mutants are lost over time due to lethal deletions and slower growth.

EMS mutant spectra generated by multi-parameter flow cytometry

The CHO A(L) cell line contains a single copy of human chromosome 11 that encodes several cell surface proteins including glycosyl phosphatidylinositol (GPI) linked CD59 and CD90, as well as CD98, CD44 and CD151 which are not GPI-linked. The flow cytometry mutation assay (FCMA) measures mutations of the CD59 gene by the absence of fluorescence when stained with antibodies against the CD59 cell surface protein. We have measured simultaneous mutations in CD59, CD44, CD90, CD98 and CD151 to generate a mutant spectrum for ionizing radiation. After treatment with ethyl methanesulfonate (EMS) many cells have an intermediate level of CD59 staining. Single cells were sorted from CD59(-) regions with varying levels of fluorescence and the resulting clonal populations had a stable phenotype for CD59 expression. Mutant spectra were generated by flow cytometry using the isolated clones and nearly all clones were mutated in CD59 only. Interestingly, about 60% of the CD59 negative clones were actually GPI mutants determined by staining with the GPI specific fluorescently labeled bacterial toxin aerolysin (FLAER). The GPI negative cells are most likely caused by mutations in the X-linked pigA gene important in GPI biosynthesis. Small mutations of pigA and CD59 were expected for the alkylating agent EMS and the resulting spectra are significantly different than the large deletions found when analyzing radiation mutants. After analyzing the CD59(-) clonal populations we have adjusted the FCMA mutant regions from 1% to 10% of the mean of the CD59 positive peak to include the majority of CD59 mutants.

Multiparameter analysis of CHO AL mutant populations sorted on CD59 expression after gamma irradiation

The flow cytometry mutation assay is based on detecting mutations in the CD59 gene on human chromosome 11 in CHO A(L) cells with flow cytometry, but the kinetics of mutant expression and the histogram region for mutant selection have not been studied in detail. CHO A(L) cells were analyzed by flow cytometry for CD59 expression at various times after irradiation. The mutant fraction increased to a maximum at day 6 but decreased to near background levels by day 20. Cells were sorted from six different regions on the CD59 histograms after irradiation. The growth rate was similar for cells from all regions, and the surviving fraction was 50% of that for control cells. By 14 days the CD59 expression of cells from regions 2-5 was reduced to that of region 1. Cells were also analyzed for simultaneous expression of CD59, CD44 and CD90 (all on chromosome 11) to roughly characterize the size of the mutations. Triple mutants from the sorted populations were reduced from 41% on day 6 to 8% on day 24. We conclude that the mutant region should be increased to include cells with intermediate CD59 expression; also, the loss of CD59 mutant expression over time could be explained in part by the loss of triple mutants from the population.

Comparison on the genotoxic effects of nuclear vs cytoplasmic irradiation from the alteration of CD59 gene locus

Using microbeam to irradiate human-hamster hybrid A(L) cells with defined number of a particles in a highly localized spatial region, this paper showed that cytoplasmic irradiation induced very little toxicity. For example, the cell killing by 4 a particle traversal through the cytoplasm was about 10%, and about 70% cells survived after their cytoplasm was irradiated with 32 a particles. In contrast, the survival fractions for nuclear irradiation at the same doses were 35% and less than 1% respectively. Mutation induction showed that while nuclear irradiation induced 3-4-fold more CD59 (-) mutants than cytoplasmic irradiation at equivalent particle traversal, at an equitoxic dose level of 90% survival, the latter exposure mode induced 3.3-fold more mutants than nuclear irradiation. Moreover, using multiplex PCR to analyze five marker genes on chromosome 11 (WT, CAT, PTH, APO-A1 and RAS), the results showed that the majority of mutants induced by cytoplasmic irradiation had retained all of the marker genes analyzed. By comparison, the proportion of mutants suffering loss of additional chromosomal markers increased with increasing number of particle traversal through nuclei.

Radiation-induced HPRT mutations resulting from misrejoined DNA double-strand breaks

DNA double-strand breaks (DSBs) are the most severe lesions induced by ionizing radiation, and unrejoined or misrejoined DSBs can lead to cell lethality, mutations and the initiation of tumorigenesis. We have investigated X-ray- and alpha-particle-induced mutations that inactivate the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene in human bladder carcinoma cells and in hTERT-immortalized human fibroblasts. Fifty to 80% of the mutants analyzed exhibited partial or total deletions of the 9 exons of the HPRT locus. The remaining mutants retained unaltered PCR products of all 9 exons but often displayed a failure to amplify the HPRT cDNA. Hybridization analysis of a 2-Mbp NotI fragment spanning the HPRT gene with a probe 200 kbp distal to the HPRT locus indicated altered fragment sizes in most of the mutants with a wild-type PCR pattern. These mutants likely contain breakpoints for genomic rearrangements in the intronic sequences of the HPRT gene that allow the amplification of the exons but prevent HPRT cDNA amplification. Additionally, mutants exhibiting partial and total deletions of the HPRT exons also frequently displayed altered NotI fragments. Interestingly, all mutations were very rarely associated with interchromosomal exchanges analyzed by FISH. Collectively, our data suggest that intrachromosomal genomic rearrangements on the Mbp scale represent the prevailing type of radiation-induced HPRT mutations.

Demonstration of a radiation-induced bystander effect for low dose low LET beta-particles

Radiation-induced bystander mutagenesis at a relatively low dose range was investigated using low LET beta-particles in a three-dimensional cell culture model. CHO cells were labeled with 0, 0.5, 1.0 or 5.0 muCi tritiated thymidine ((3)HdTTP) for 12 h and subsequently incubated with A(L) cells for 24 h at 11 degrees C. The cell mixture was centrifuged to produce a spheroid of 4 x 10(6) cells of which there was five times more A(L) than CHO cells. The short-range beta-particles emitted by (3)HdTTP result in self-irradiation of labeled CHO cells, thus biological effects on neighboring A(L) cells can be attributed to the bystander response. To evaluate such response, non-labeled bystander A(L) cells were isolated from among labeled CHO cells and studied independently for survival and mutagenesis. Treatment of CHO cells with (3)HdTTP resulted in a dose-dependent increase in bystander mutation incidence among neighboring A(L) cells compared to controls. In addition, multiplex PCR analysis revealed the types of mutants to be significantly different from those of spontaneous origin. These data provide evidence that low dose low LET radiation can induce bystander mutagenesis in a three-dimensional model. The results of this study will address the relevant issues of actual target size and radiation quality, and are likely to have a significant impact on our current understanding of radiation risk assessment.

Mutant spectra of irradiated CHO AL cells determined with multiple markers analyzed by flow cytometry

We have previously developed a sensitive and rapid mammalian cell mutation assay which is based on a Chinese hamster ovary cell line that stably incorporates human chromosome 11 (CHO A(L)) and uses flow cytometry to measure mutations in CD59. We now show that multiparameter flow cytometry may be used to simultaneously analyze irradiated CHO A(L) cells for mutations in five CD genes along chromosome 11 (CD59, CD44, CD90, CD98, CD151) and also a GPI-anchor gene. Using this approach, 19 different mutant clones derived from individual sorted mutant cells were analyzed to determine the mutant spectrum induced by ionizing radiation. All clones analyzed were negative for CD59 expression and PCR confirmed that at least CD59 exon 4 was also absent. As expected, ionizing radiation frequently caused large deletions along chromosome 11. This technology can readily be used to rapidly analyze the mutant yield as well as the spectrum of mutations caused by a variety of genotoxic agents and provide greater insight into the mechanisms of mutagenesis.

Comparison of the mutagenic potential of 17 physical and chemical agents analyzed by the flow cytometry mutation assay

Several methods to assess genotoxicity of physical and chemical agents have been developed, most of which depend on growing colonies in selective medium. We recently published a new method for detecting mutations in the CD59 gene in a Chinese hamster ovary cell line that contains a single copy of human chromosome 11 (CHO A(L)). The assay is based on detecting the surface expression of CD59 with monoclonal antibodies using flow cytometry. The capabilities of this flow cytometry mutation assay (FCMA) to detect mutations from a wide variety of genotoxic agents are described here. There was a 400-fold separation between CD59- and CD59+ populations based on fluorescence intensity. Small numbers of negative cells mixed in with positive cells were detected in a highly linear fashion. Mutation dose response curves over a dose range yielding 80% to 20% survival are shown for ethyl methane sulfonate (EMS), mitomycin C (MMC) and lead acetate. EMS and lead acetate exhibited a threshold in response while MMC had a linear dose response over the full dose range. The mutant fraction was measured over time periods ranging up to 35 days following treatment. The mutant fraction peaked at different times ranging from 6 to 12 days after treatment. An additional 14 chemical and physical agents including point mutagens, heavy metals, ionizing and UV radiation, and DNA intercalators and cross linkers, were analyzed for mutagenic potential after doses giving 80% to 20% survival. The results presented here demonstrate the sensitivity and broad-ranging capability of the FCMA to detect mutations induced by a variety of genotoxic agents.

Studies on chromosome aberration induction: What can they tell us about DNA repair?

Many, if not the majority of spontaneous or induced mutations in somatic mammalian cells associated with cancer are large chromosome level changes. For exposure to carcinogenic agents, certain specific chromosomal aberrations are likely to lie early along the pathway leading from initial molecular damage to cancer. The kinds of aberrations that occur, and the positions of breakpoints involved in their formation, can reveal not only genes and controlling elements whose expression or suppression underlie the molecular nature of the initiation of malignant transformation, but also how structural and functional features of chromatin can affect processes involved in repair or mis-repair of initial DNA damage. Thus, cytogenetics can provide information in ways that are not readily appreciated in studies requiring disruption of chromatin organization as it exists in the cell and its tissue context, and where DNA repair assays measure effects averaged over the entire genome. Examples include the fact that in contrast to a more efficient repair of single strand or base damage in transcriptionally active chromatin, after ionizing radiation exposure, the preponderance of translocation breakpoints indicating mis-repair occur in transcriptionally active or potentially active chromatin. Cytogenetic studies have led to the recognition that processing of DNA ends - both ends resulting from breaks along chromosomes and natural chromosomal termini, or telomeres - share very interesting similarities and differences. Further, direct observation of chromatin in cells during interphase can speak directly to early stages of aberration formation where processes occur within the context of intact cells, and to the role (or lack thereof) of cell cycle checkpoint responses that often accompany DNA damage. The superior resolution of many of the current molecular cytogenetics approaches, combined with immunocytochemical detection of proteins involved in DNA damage processing, and the availability of repair deficient mutants or knockdown strategies such as RNA interference, suggest that cytogenetics may still provide useful information and set certain restrictions important for rational interpretation of studies of DNA repair and associated protein interactions that can only be carried out in vitro. The intent of this paper is to focus on contributions of studies on the production of chromosomal aberrations following ionizing radiation exposure regarding important insights on associated DNA repair processes involved, and further, on guidelines or constraints they provide for the interpretation of in vitro DNA repair studies that would have been difficult to appreciate without the cytogenetics. We will first briefly summarize some early studies that serve as a reminder of the background on which current studies are based, and then carry forward to the present day certain interesting facets of these studies.

Molecular mechanisms of cadmium induced mutagenicity

Cadmium is a human carcinogen of worldwide concern because it accumulates in the environment due to its extremely long half-life. Its compounds are classified as human carcinogens by several regulatory agencies. Cadmium affects cell proliferation, differentiation, apoptosis and other cellular activities and can cause numerous molecular lesions that would be relevant to carcinogenesis. For a long time cadmium has been considered as a non-genotoxic carcinogen, as it is only weakly mutagenic in bacterial and mammalian cell test systems. Recently, we presented evidence that when assayed in a test system, in which both intragenic and multilocus mutations can be detected, cadmium acts as a strong mutagen which induces predominantly multilocus deletions. In this review, we discuss two mechanisms that play an important role in cadmium mutagenicity: (i) induction of reactive oxygen species (ROS); and (ii) inhibition of DNA repair. Experimental evidence suggests that cadmium at low, for environmental exposure relevant concentrations, induces mutations by inducing oxidative DNA damage and that it decreases genetic stability by inhibiting the repair of endogenous and exogenous DNA lesions, which in turn increase the probability of mutations and consequently cancer initiation by this metal.

Assessment of low linear energy transfer radiation-induced bystander mutagenesis in a three-dimensional culture model

A three-dimensional cell culture model composed of human-hamster hybrid (A(L)) and Chinese hamster ovary (CHO) cells in multicellular clusters was used to investigate low linear energy transfer (LET) radiation-induced bystander genotoxicity. CHO cells were mixed with A(L) cells in a 1:5 ratio and briefly centrifuged to produce a spheroid of 4 x 10(6) cells. CHO cells were labeled with tritiated thymidine ([3H]dTTP) for 12 hours and subsequently incubated with A(L) cells for 24 hours at 11 degrees C. The short-range beta-particles emitted by [3H]dTTP result in self-irradiation of labeled CHO cells; thus, biological effects on neighboring A(L) cells can be attributed to the bystander response. Nonlabeled bystander A(L) cells were isolated from among labeled CHO cells by using a magnetic separation technique. Treatment of CHO cells with 100 microCi [3H]dTTP resulted in a 14-fold increase in bystander mutation incidence among neighboring A(L) cells compared with controls. Multiplex PCR analysis revealed the types of mutants to be significantly different from those of spontaneous origin. The free radical scavenger DMSO or the gap junction inhibitor Lindane within the clusters significantly reduced the mutation incidence. The use of A(L) cells that are dominant negative for connexin 43 and lack gap junction formation produced a complete attenuation of the bystander mutagenic response. These data provide evidence that low LET radiation can induce bystander mutagenesis in a three-dimensional model and that reactive oxygen species and intercellular communication may have a modulating role. The results of this study will address the relevant issues of actual target size and radiation quality and are likely to have a significant effect on our current understanding of radiation risk assessment.

Quantification of CD59- mutants in human-hamster hybrid (AL) cells by flow cytometry

Mutation assay is an important approach in evaluating the genotoxic risk of potentially harmful environmental chemicals. The human-hamster hybrid (A(L)) cell mutagenesis system, based on the complement/antibody-mediated cytotoxicity principle, has been used successfully to evaluate the mutagenic potential of a variety of environmental toxicants. The A(L) cells contain a standard set of CHO chromosomes and a single human chromosome 11, which expresses several cell surface proteins including CD59 encoded by the CD59 gene at 11p13.5. A modified mutation assay by flow cytometry was developed to determine the yield of CD59- mutants after either radiation or chemical treatment. After incubation with phycoerythrin-conjugated mouse monoclonal anti-CD59 antibody, the CD59- mutant yields were determined by quantifying the fluorescence of the cells using flow cytometry. This method is faster and eliminates the commonly encountered toxicity problems of the complements with the traditional complement/antibody assay. By comparing the mutant fractions of radiation or chemically treated A(L) cultures using the two methods, we show here that the flow cytometry assay is an excellent substitute in providing an efficient and highly sensitive method in mutant detection for the traditional complement/antibody assay.

Assay to measure CD59 mutations in CHO A(L) cells using flow cytometry

BACKGROUND

A sensitive mammalian cell mutation assay was developed previously using a Chinese hamster ovary cell line (CHO A(L)) that stably incorporates human chromosome 11. The assay measures mutations in the CD59 gene on chromosome 11 but it requires the use of rabbit complement and colony growth for mutant selection. We have developed a more rapid flow cytometry-based mutation assay with CHO A(L) cells that uses monoclonal antibodies against CD59 to detect mutants and does not require colony formation.

METHODS

CHO A(L) cells were treated with gamma-radiation or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and then allowed to grow for various times for mutant expression. Cells were labeled with monoclonal antibodies against CD59 and analyzed by flow cytometry.

RESULTS

Negative and positive populations were separated by over 100-fold. Mixing various proportions of CD59-positive and -negative cells demonstrated that the assay is highly linear (r2 = 0.9999) and sensitive (<0.05% background mutants). The yield of CD59-inducible mutants was linearly related to dose for a clastogen (gamma-radiation) and point mutagen (MNNG). The mutant yield was time and treatment specific.

CONCLUSIONS

Mutations induced by genotoxic agents can be rapidly and sensitively measured in CHO A(L) cells using flow cytometry.

Mitochondrial Damage Mediates Genotoxicity of Arsenic in Mammalian Cells

Arsenic is an important environmental carcinogen that affects millions of people worldwide through contaminated water supplies. For decades, arsenic was considered a nongenotoxic carcinogen. Using the highly sensitive A(L) mutation assay, we previously showed that arsenic is, indeed, a potent gene and chromosomal mutagen and that its effects are mediated through the induction of reactive oxygen species. However, the origin of these radicals and the pathways involved are not known. Here we show that mitochondrial damage plays a crucial role in arsenic mutagenicity. Treatment of enucleated cells with arsenic followed by rescue fusion with karyoplasts from controls resulted in significant mutant induction. In contrast, treatment of mitochondrial DNA-depleted (rho(0)) cells produced few or no mutations. Mitochondrial damage can lead to the release of superoxide anions, which then react with nitric oxide to produce the highly reactive peroxynitrites. The mutagenic damage was dampened by the nitric oxide synthase inhibitor, N(G)-methyl-L-arginine. These data illustrate that mitochondria are a primary target in arsenic-induced genotoxic response and that a better understanding of the mutagenic/carcinogenic mechanism of arsenic should provide a basis for better interventional approach in both treatment and prevention of arsenic-induced cancer.

Genotoxicity in the eyes of bystander cells

The controversial use of a linear, no threshold extrapolation model for low dose risk assessment has become even more so in light of the recent reports on the bystander phenomenon. The answer to the question as to which of the two phenomena, bystander versus adaptive response, is more important has practical implication in terms of low dose radiation risk assessment. In this review, genotoxicity is used as an endpoint to introduce the two phenomena, provide some insight into the mechanisms of bystander effect and to bridge the two low dose phenomena which operate in opposite directions: the bystander effect tends to exaggerate the effect at low doses, by communicating damage from hit to non-hit cells whereas the adaptive response confers resistance to a subsequent challenging dose by an initial low priming dose.

A role for long-lived radicals (LLR) in radiation-induced mutation and persistent chromosomal instability: counteraction by ascorbate and RibCys but not DMSO

Miazaki, Watanabe, Kumagai and their colleagues reported that induction of HPRT(-) mutants by X-rays in cultured human cells was prevented by ascorbate added 30min after irradiation. They attributed extinction of induced mutation to neutralization by ascorbate of radiation-induced long-lived mutagenic radicals (LLR), found using spectroscopy to have half-lives of minutes or hours. We find that post-irradiation treatment with ascorbate reduces, but does not eliminate, induction of CD59(-) mutants in human-hamster hybrid A(L) cells exposed to high-LET carbon-ions (LET of 100KeV/microm). A(L) cells contain a standard set of Chinese hamster ovary (CHO) chromosomes and a single copy of human chromosome 11 containing the CD59 gene which encodes the CD59 cell surface antigen, a convenient marker for mutation. RibCys [2(R, S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)thiazolidine-4(R)-carboxylic acid] a 'prodrug' of l-cysteine which also scavenges LLR, had a similar but lesser effect on induced mutation. DMSO, which scavenges classical radicals like H* and OH* but not LLR, also reduced mutation, but only when it was present during irradiation. The lethality of carbon-ions was not altered by ascorbate, RibCys no matter when added. Post-radiation addition of ascorbate and RibCys also affected the quality of CD59(-) mutations induced by carbon-ions. The major change in mutant spectra was a reduction in the prevalence of small, intragenic mutations (mutations not detected by PCR) and in the prevalence of unstable, complicated mutants, which display high levels of persistent chromosomal instability. Thus, ascorbate and RibCys may suppress some kinds of mutations induced by ionizing radiation including those displaying aspects of radiation-induced genomic instability. Countering the effects of both classical radicals and LLR may be important in preventing genetic diseases.

Interaction between radiation-induced adaptive response and bystander mutagenesis in mammalian cells

Two conflicting phenomena, the bystander effect and the adaptive response, are important in determining biological responses at low doses of radiation and have the potential to have an impact on the shape of the dose-response relationship. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, we reported previously that nonirradiated cells acquired mutagenesis through direct contact with cells whose nuclei had previously been traversed with either a single or 20 alpha particles each. Here we show that pretreatment of cells with a low dose of X rays 4 h before alpha-particle irradiation significantly decreased this bystander mutagenic response. Furthermore, bystander cells showed an increase in sensitivity after a subsequent challenging dose of X rays. Results from the present study address some of the pressing issues regarding both the actual target size and the radiation dose response and can improve on our current understanding of radiation risk assessment.

Radiation-induced bystander effect and adaptive response in mammalian cells

Two conflicting phenomena, bystander effect and adaptive response, are important in determining the biological responses at low doses of radiation and have the potential to impact the shape of the dose-response relationship. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, we show here that non-irradiated cells acquire mutagenesis through direct contact with cells whose nuclei have been traversed with a single alpha particle each. Pretreatment of cells with a low dose of X-rays four hours before alpha particle irradiation significantly decreased this bystander mutagenic response. Results from the present study address some of the fundamental issues regarding both the actual target and radiation dose effect and can contribute to our current understanding in radiation risk assessment.

Mutagenic effect of low energy neutrons on human chromosome 11

PURPOSE

The shape of the dose-effect curve for neutrons, i.e. the question as to whether the curve is linear or supralinear in the low-dose region, is still not clear. Therefore, the mutagenic effect of very low doses of low-energy neutrons was determined.

MATERIALS AND METHODS

Human-hamster hybrid A(L) cells contain human chromosome 11, which expresses the membrane protein CD59. This membrane protein can be detected immunologically and quantified by flow cytometry. The A(L) cells were irradiated with neutrons of 0.565, 2.5 or 14.8 MeV and the results were compared with those after 200 kVp X-rays. Before irradiation, cells spontaneously mutated in the CD59 gene were removed by magnetic cell sorting (MACS).

RESULTS

The relative biological effectiveness (RBE) for CD59 mutation induction was 19.8 (+/-2.7) for 0.565 MeV, 10.2 (+/-1.9) for 2.5 MeV, and 10.2 (+/-1.6) for 14.8 MeV neutrons. Linear mutation responses were obtained with all radiations except for 14.8 MeV neutrons where a supralinear curve may be a better fit. The deletion spectrum of mutated cell clones showed 29 Mbp deletions on average after irradiation with 0.069 Gy of 0.565 MeV neutrons. This scale of deletions is similar to that after 3 Gy 100 kV X-rays (=34 Mbp). For 50% cell survival, the RBE of the neutrons was 11 compared with 200 kV X-rays.

CONCLUSIONS

Neutrons of low energies (0.565 or 2.5 MeV) produce a linear dose-response for mutation in the tested dose range of 0.015-0.15 Gy. The neutron curve of 14.8 MeV can be approximated by a curvilinear or linear function.

The “Pro-drug” RibCys Decreases the Mutagenicity of High-LET Radiation in Cultured Mammalian Cells

We are carrying out studies aimed at reducing the mutagenic effects of high-LET 56Fe ions and 12C ions (56Fe ions, 143 keV/microm; 12C ions, 100 keV/microm) with certain drugs, including RibCys [2-(R,S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)-thiazolidine-4(R)-carboxylic acid]. RibCys, formed by condensation of L-cysteine with D-ribose, is designed so that the sulfhydryl amino acid L-cysteine is released intracellularly through nonenzymatic ring opening and hydrolysis leading to increased levels of glutathione (GSH). RibCys (4 or 10 mM), which was present during irradiation and for a few hours after, significantly decreased the yield of CD59- mutants induced by radiation in AL human-hamster hybrid cells. RibCys did not affect the clonogenic survival of irradiated cells, nor was it mutagenic itself. These results, together with the minimal side effects reported in mice and pigs, indicate that RibCys may be useful, perhaps even when used prophylactically, in reducing the mutation load created by high-LET radiation in astronauts or other exposed individuals.

Applications of RNA interference for studies in DNA damage processing, genome stability, mutagenesis, and cancer

RNA interference, discovered only five years ago, is an important system for the control of gene expression. It is also quickly becoming a valuable experimental tool as it allows investigators to knock down the level of expression of specific genes. In this paper, we review some applications of this technology for studies in DNA damage processing, genome stability, mutagenesis, and cancer.

Mutation induction and neoplastic transformation in human and human-hamster hybrid cells: dependence on photon energy and modulation in the low-dose range

Mutation induction in the HPRT gene of human fibroblasts after irradiation with mammography-like 29 kVp or 200 kVp x-rays shows radiohypersensitivity for doses smaller than approximately 0.5 Gy. Similarly, mutation induction in the CD 59 gene on human chromosome 11 in A(L) cells shows radiohypersensitivity for doses smaller than approximately 0.5 Gy after exposure to 200 kVp x-rays, but not after irradiation with low-filtered 30 kVp x-rays. The RBE values of 29 and 30 kVp x-rays relative to 200 kVp x-rays are strongly dose dependent. For neoplastic transformation of human hybrid (CGL1) cells after irradiation with 29 or 200 kVp x-rays or 60Co gamma rays a linear-quadratic dose relationship was observed with RBE values of approximately four and eight for mammography relative to 200 kVp x-rays and 60Co gamma rays, respectively.

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

Arsenic induces oxidative DNA damage in mammalian cells

Although arsenic is a well-established human carcinogen, the underlying carcinogenic mechanism(s) is not known. Using the human-hamster hybrid (A(L)) cell mutagenic assay that is sensitive in detecting mutagens that induce predominately multilocus deletions, we showed previously that arsenite is indeed a potent gene and chromosomal mutagen and that oxyradicals may be involved in the mutagenic process. In the present study, the effects of free radical scavenging enzymes on the cytotoxic and mutagenic potential of arsenic were examined using the AL cells. Concurrent treatment of cells with either superoxide dismutase or catalase reduced both the cytotoxicity and mutagenicity of arsenite by an average of 2-3 fold, respectively. Using immunoperoxidase staining with a monoclonal antibody specific for 8-hydroxy-2'-deoxyguanosine (8-OHdG), we demonstrated that arsenic induced oxidative DNA damage in A(L) cells. This induction was significantly reduced in the presence of the antioxidant enzymes. Furthermore, reducing the intracellular levels of non-protein sulfhydryls (mainly glutathione) using buthionine S-R-Sulfoximine increased the total mutant yield by more than 3-fold as well as the proportion of mutants with multilocus deletions. Taken together, our data provide clear evidence that reactive oxygen species play an important causal role in the genotoxicity of arsenic in mammalian cells.

Effects of irradiated medium with or without cells on bystander cell responses

Recent studies have indicated that extranuclear or extracellular targets are important in mediating the bystander genotoxic effects of alpha-particles. In the present study, human-hamster hybrid (A(L)) cells were plated on either one or both sides of double-mylar dishes 2-4 days before irradiation, depending on the density requirement of experiments. One side (with or without cells) was irradiated with alpha-particles (from 0.1 to 100 Gy) using the track segment mode of a 4 MeV Van de Graaff accelerator. After irradiation, cells were kept in the dishes for either 1 or 48 h. The non-irradiated cells were then collected and assayed for both survival and mutation. When one side with cells was irradiated by alpha-particles (1, 10 and 100 Gy), the surviving fraction among the non-irradiated cells was significantly lower than that of control after 48 h co-culture. However, such a change was not detected after 1h co-culture or when medium alone was irradiated. Furthermore, co-cultivation with irradiated cells had no significant effect on the spontaneous mutagenic yield of non-irradiated cells collected from the other half of the double-mylar dishes. These results suggested that irradiated cells released certain cytotoxic factor(s) into the culture medium that killed the non-irradiated cells. However, such factor(s) had little effect on mutation induction. Our results suggest that different bystander end points may involve different mechanisms with different cell types.

Radiation risk to low fluences of alpha particles may be greater than we thought

Based principally on the cancer incidence found in survivors of the atomic bombs dropped in Hiroshima and Nagasaki, the International Commission on Radiation Protection (ICRP) and the United States National Council on Radiation Protection and Measurements (NCRP) have recommended that estimates of cancer risk for low dose exposure be extrapolated from higher doses by using a linear, no-threshold model. This recommendation is based on the dogma that the DNA of the nucleus is the main target for radiation-induced genotoxicity and, as fewer cells are directly damaged, the deleterious effects of radiation proportionally decline. In this paper, we used a precision microbeam to target an exact fraction (either 100% or < or =20%) of the cells in a confluent population and irradiated their nuclei with exactly one alpha particle each. We found that the frequencies of induced mutations and chromosomal changes in populations where some known fractions of nuclei were hit are consistent with non-hit cells contributing significantly to the response. In fact, irradiation of 10% of a confluent mammalian cell population with a single alpha particle per cell results in a mutant yield similar to that observed when all of the cells in the population are irradiated. This effect was significantly eliminated in cells pretreated with a 1 mM dose of octanol, which inhibits gap junction-mediated intercellular communication, or in cells carrying a dominant negative connexin 43 vector. The data imply that the relevant target for radiation mutagenesis is larger than an individual cell and suggest a need to reconsider the validity of the linear extrapolation in making risk estimates for low dose, high linear-energy-transfer (LET) radiation exposure.

Large-mutation spectra induced at hemizygous loci by low-LET radiation: evidence for intrachromosomal proximity effects

A mathematical model is used to analyze mutant spectra for large mutations induced by low-LET radiation. The model equations are based mainly on two-break misrejoining that leads to deletions or translocations. It is assumed, as a working hypothesis, that the initial damage induced by low-LET radiation is located randomly in the genome. Specifically, we analyzed data for two hemizygous loci: CD59- mutants, mainly very large-scale deletions (>3 Mbp), in human-hamster hybrid cells, and data from the literature on those HPRT- mutants which involve at least deletion of the whole gene, and often of additional flanking markers (approximately 50-kbp to approximately 4.4-Mbp deletions). For five data sets, we estimated f, the probability that two given breaks on the same chromosome will misrejoin to make a deletion, as a function of the separation between the breaks. We found that f is larger for nearby breaks than for breaks that are more widely separated; i.e., there is a "proximity effect". For acute irradiation, the values of f determined from the data are consistent with the corresponding break misrejoining parameters found previously in quantitative modeling of chromosome aberrations. The value of f was somewhat smaller for protracted irradiation than for acute irradiation at a given total dose; i.e., the mutation data show a decrease that was smaller than expected for dose protraction by fractionation or low dose rate.

Gamma-ray mutagenesis studies in a new human-hamster hybrid, A(L)CD59(+/-), which has two human chromosomes 11 but is hemizygous for the CD59 gene

Kraemer, S. M., Vannais, D. B., Kronenberg, A., Ueno, A. and Waldren, C. A. Gamma-Ray Mutagenesis Studies in a New Human-Hamster Hybrid, A(L)CD59(+/-), which has Two Human Chromosomes 11 but is Hemizygous for the CD59 Gene. Radiat. Res. 156, 10-19 (2001). We have developed a human-CHO hybrid cell line, named A(L)CD59(+/-), which has two copies of human chromosome 11 but is hemizygous for the CD59 gene and the CD59 cell surface antigen that it encodes. Our previous studies used the A(L) and A(L)C hybrids that respectively contain one or two sets of CHO chromosomes plus a single copy of human chromosome 11. The CD59 gene at 11p13.5 and the CD59 antigen encoded by it are the principal markers used in our mutagenesis studies. The hybrid A(L)CD59(+/-) contains two copies of human chromosome 11, only one of which carries the CD59 gene. The incidence of CD59 (-) mutants (formerly called S1(-)) induced by (137)Cs gamma rays is about fivefold greater in A(L)CD59(+/-) cells than in A(L) cells. Evidence is presented that this increase in mutant yield is due to the increased induction of certain classes of large chromosomal mutations that are lethal to A(L) cells but are tolerated in the A(L)CD59(+/-) hybrid. In addition, significantly more of the CD59 (-) mutants induced by (137)Cs gamma rays in A(L)CD59(+/-) cells display chromosomal instability than in A(L) cells. On the other hand, the yield of gamma-ray-induced CD59 (-) mutants in A(L)CD59(+/-) cells is half that of the A(L)C hybrid, which also tolerates very large mutations but has only one copy of human chromosome 11. We interpret the difference in mutability as evidence that repair processes involving the homologous chromosomes 11 play a role in determining mutant yields. The A(L)CD59(+/-) hybrid provides a useful new tool for quantifying mutagenesis and shedding light on mechanisms of genetic instability and mutagenesis.

The role of glutathione in the toxicity of smoke condensates from cigarettes that burn or heat tobacco

Inhalation of cigarette smoke aerosol via active smoking is associated with the development of pulmonary inflammation. The cytotoxic potential of cigarette smoke has been hypothetically related to development of pulmonary inflammation since the release of intracellular contents from dead and dying cells has been reported to induce inflammatory foci. In this study, cigarette smoke condensates (CSCs) were prepared from Kentucky 1R4F reference cigarettes and cigarettes that primarily heat tobacco (Eclipse). The two CSCs were then compared for their ability to induce killing in human-hamster A(L) hybrid cells. CSCs prepared from Eclipse were much less cytotoxic than those prepared from reference cigarettes. At 60 microg CSC/ml culture medium, survival for CSC from Eclipse cigarettes was approximately 70% compared with 1% for CSC from burned K1R4F cigarettes. The observed reduction in CSC-Eclipse cytotoxicity toward these mammalian cells is consistent with the previously published observation of a 30% decline in pulmonary white cell count and 40% reduction in visual bronchitis index in human smokers who switched to Eclipse for 2 months. Results with N-acetylcysteine and buthionine-S-R-sulfoximine indicate that glutathione markedly reduces the cytoxicity of both CSCs.

Induction of oxyradicals by arsenic: implication for mechanism of genotoxicity

Although arsenic is a well-established human carcinogen, the mechanisms by which it induces cancer remain poorly understood. We previously showed arsenite to be a potent mutagen in human-hamster hybrid (A(L)) cells, and that it induces predominantly multilocus deletions. We show here by confocal scanning microscopy with the fluorescent probe 5',6'-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate that arsenite induces, within 5 min after treatment, a dose-dependent increase of up to 3-fold in intracellular oxyradical production. Concurrent treatment of cells with arsenite and the radical scavenger DMSO reduced the fluorescent intensity to control levels. ESR spectroscopy with 4-hydroxy-2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOL-H) as a probe in conjunction with superoxide dismutase and catalase to quench superoxide anions and hydrogen peroxide, respectively, indicates that arsenite increases the levels of superoxide-driven hydroxyl radicals in these cells. Furthermore, reducing the intracellular levels of nonprotein sulfhydryls (mainly glutathione) in A(L) cells with buthionine S-R-sulfoximine increases the mutagenic potential of arsenite by more than 5-fold. The data are consistent with our previous results with the radical scavenger DMSO, which reduced the mutagenicity of arsenic in these cells, and provide convincing evidence that reactive oxygen species, particularly hydroxyl radicals, play an important causal role in the genotoxicity of arsenical compounds in mammalian cells.

Induction of oxyradicals by arsenic: implication for mechanism of genotoxicity

Although arsenic is a well-established human carcinogen, the mechanisms by which it induces cancer remain poorly understood. We previously showed arsenite to be a potent mutagen in human-hamster hybrid (A(L)) cells, and that it induces predominantly multilocus deletions. We show here by confocal scanning microscopy with the fluorescent probe 5',6'-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate that arsenite induces, within 5 min after treatment, a dose-dependent increase of up to 3-fold in intracellular oxyradical production. Concurrent treatment of cells with arsenite and the radical scavenger DMSO reduced the fluorescent intensity to control levels. ESR spectroscopy with 4-hydroxy-2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOL-H) as a probe in conjunction with superoxide dismutase and catalase to quench superoxide anions and hydrogen peroxide, respectively, indicates that arsenite increases the levels of superoxide-driven hydroxyl radicals in these cells. Furthermore, reducing the intracellular levels of nonprotein sulfhydryls (mainly glutathione) in A(L) cells with buthionine S-R-sulfoximine increases the mutagenic potential of arsenite by more than 5-fold. The data are consistent with our previous results with the radical scavenger DMSO, which reduced the mutagenicity of arsenic in these cells, and provide convincing evidence that reactive oxygen species, particularly hydroxyl radicals, play an important causal role in the genotoxicity of arsenical compounds in mammalian cells.

Frequency of CD59 mutations induced in human-hamster hybrid A(L) cells by low-dose X-irradiation

Determination of the genotoxic effects of ionizing radiation, especially at low-doses, is of great importance for risk assessment, e.g. in radiological diagnostics. The human-hamster hybrid A(L) cell line has been shown previously to be a well-suited in vitro model for the study of mutations induced by various mutagens. The A(L) cells contain a standard set of hamster chromosomes and a single human chromosome 11, which confers the expression of the human cell surface protein CD59. Using CD59 specific antibodies, cells mutated in the CD59 gene can be detected and quantified by the loss of the cell surface marker. In contrast to previous studies, prior to irradiation we removed spontaneous mutants by magnetic cell separation (MACS) which allows analysis of radiation-induced mutation events only. We exposed A(L) cells to 100kV X-rays at 0.1 to 5Gy. The proportions of X-irradiation-induced CD59(-) mutants were quantified by flow cytometry after immunofluorescence labeling. Between 0.2 and 5Gy the yield of CD59 mutants was a linear function of dose. The molecular analysis of individual CD59-negative clones induced after exposure of 1, 3 and 5Gy of X-ray revealed a dose-dependent linear increase of large deletions (>6Mbp), whereas, point mutations could be seen only in spontaneous CD59 mutants or after low-dose exposure (< or =1Gy). We conclude that the modified A(L) assay presented here is appropriate for detection and quantification of non-lethal DNA lesions induced by low-dose ionizing radiation.