Effect of metals on mutagenesis and DNA repair

Effects of copper, and aluminium in ionic, and nanoparticulate form on growth rate and gene expression of Setaria italica seedlings

This study aims to determine the effects of copper, copper oxide nanoparticles, aluminium, and aluminium oxide nanoparticles on the growth rate and expression of ACT-1, CDPK, LIP, NFC, P5CR, P5CS, GR, and SiZIP1 genes in five days old seedling of Setaria italica ssp. maxima, cultivated in hydroponic culture. Depending on their concentration (ranging from 0.1 to 1.8 mg L-1), all tested substances had both stimulating and inhibiting effects on the growth rate of the seedlings. Copper and copper oxide-NPs had generally a stimulating effect whereas aluminium and aluminium oxide-NPs at first had a positive effect but in higher concentrations they inhibited the growth. Treating the seedlings with 0.4 mg L-1 of each tested toxicant was mostly stimulating to the expression of the genes and reduced the differences between the transcript levels of the coleoptiles and roots. Increasing concentrations of the tested substances had both stimulating and inhibiting effects on the expression levels of the genes. The highest expression levels were usually noted at concentrations between 0.4 and 1.0 mg/L of each metal and metal nanoparticle, except for SiZIP1, which had the highest transcript amount at 1.6 mg L-1 of Cu2+ and at 0.1-0.8 mg L-1 of CuO-NPs, and LIP and GR from the seedling treated with Al2O3-NPs at concentrations of 0.1 and 1.6 mg L-1, respectively.

Hydroxyl Radical Overproduction in the Envelope: an Achilles' Heel in Peptidoglycan Synthesis

While many mechanisms governing bacterial envelope homeostasis have been identified, others remain poorly understood. To decipher these processes, we previously developed an assay in the Gram-negative model Escherichia coli to identify genes involved in maintenance of envelope integrity. One such gene was ElyC, which was shown to be required for envelope integrity and peptidoglycan synthesis at room temperature. ElyC is predicted to be an integral inner membrane protein with a highly conserved domain of unknown function (DUF218). In this study, and stemming from a further characterization of the role of ElyC in maintaining cell envelope integrity, we serendipitously discovered an unappreciated form of oxidative stress in the bacterial envelope. We found that cells lacking ElyC overproduce hydroxyl radicals (HO^•) in their envelope compartment and that HO^• overproduction is directly or indirectly responsible for the peptidoglycan synthesis arrest, cell envelope integrity defects, and cell lysis of the ΔelyC mutant. Consistent with these observations, we show that the ΔelyC mutant defect is suppressed during anaerobiosis. HO^• is known to cause DNA damage but to our knowledge has not been shown to interfere with peptidoglycan synthesis. Thus, our work implicates oxidative stress as an important stressor in the bacterial cell envelope and opens the door to future studies deciphering the mechanisms that render peptidoglycan synthesis sensitive to oxidative stress. IMPORTANCE Oxidative stress is caused by the production and excessive accumulation of oxygen reactive species. In bacterial cells, oxidative stress mediated by hydroxyl radicals is typically associated with DNA damage in the cytoplasm. Here, we reveal the existence of a pathway for oxidative stress in the envelope of Gram-negative bacteria. Stemming from the characterization of a poorly characterized gene, we found that HO^• overproduction specifically in the envelope compartment causes inhibition of peptidoglycan synthesis and eventually bacterial cell lysis.

Urinary Arsenic in Human Samples from Areas Characterized by Natural or Anthropogenic Pollution in Italy

Arsenic is ubiquitous and has a potentially adverse impact on human health. We compared the distribution of concentrations of urinary inorganic arsenic plus methylated forms (uc(iAs+MMA+DMA)) in four Italian areas with other international studies, and we assessed the relationship between uc(iAs+MMA+DMA) and various exposure factors. We conducted a human biomonitoring study on 271 subjects (132 men) aged 20-44, randomly sampled and stratified by area, gender, and age. Data on environmental and occupational exposure and dietary habits were collected through a questionnaire. Arsenic was speciated using chromatographic separation and inductively coupled mass spectrometry. Associations between uc(iAs+MMA+DMA) and exposure factors were evaluated using the geometric mean ratio (GMR) with a 90% confidence interval by stepwise multiple regression analysis. The 95th percentile value of uc(iAs+MMA+DMA) for the whole sample (86.28 µg/L) was higher than other national studies worldwide. A statistical significant correlation was found between uc(iAs+MMA+DMA) and occupational exposure (GMR: 2.68 [1.79-4.00]), GSTT gene (GMR: 0.68 [0.52-0.80]), consumption of tap water (GMR: 1.35 [1.02-1.77]), seafood (GMR: 1.44 [1.11-1.88]), whole milk (GMR: 1.34 [1.04-1.73]), and fruit/vegetables (GMR: 1.37 [1.03-1.82]). This study demonstrated the utility of uc(iAs+MMA+DMA) as a biomarker to assess environmental exposure. In a public health context, this information could be used to support remedial action, to prevent individuals from being further exposed to environmental arsenic sources.

Genotoxic and epigenetic mechanisms in arsenic carcinogenicity

Arsenic is a human carcinogen with weak mutagenic properties that induces tumors through mechanisms not yet completely understood. People worldwide are exposed to arsenic-contaminated drinking water, and epidemiological studies showed a high percentage of lung, bladder, liver, and kidney cancer in these populations. Several mechanisms by which arsenical compounds induce tumorigenesis were proposed including genotoxic damage and chromosomal abnormalities. Over the past decade, a growing body of evidence indicated that epigenetic modifications have a role in arsenic-inducing adverse effects on human health. The main epigenetic mechanisms are DNA methylation in gene promoter regions that regulate gene expression, histone tail modifications that regulate the accessibility of transcriptional machinery to genes, and microRNA activity (noncoding RNA able to modulate mRNA translation). The "double capacity" of arsenic to induce mutations and epimutations could be the main cause of arsenic-induced carcinogenesis. The aim of this review is to better clarify the mechanisms of the initiation and/or the promotion of arsenic-induced carcinogenesis in order to understand the best way to perform an early diagnosis and a prompt prevention that is the key point for protecting arsenic-exposed population. Studies on arsenic-exposed population should be designed in order to examine more comprehensively the presence and consequences of these genetic/epigenetic alterations.

ATF4-dependent oxidative induction of the DNA repair enzyme Ape1 counteracts arsenite cytotoxicity and suppresses arsenite-mediated mutagenesis

Arsenite is a human carcinogen causing skin, bladder, and lung tumors, but the cellular mechanisms underlying these effects remain unclear. We investigated expression of the essential base excision DNA repair enzyme apurinic endonuclease 1 (Ape1) in response to sodium arsenite. In mouse 10T(1/2) fibroblasts, Ape1 induction in response to arsenite occurred about equally at the mRNA, protein, and enzyme activity levels. Analysis of the APE1 promoter region revealed an AP-1/CREB binding site essential for arsenite-induced transcriptional activation in both mouse and human cells. Electrophoretic mobility shift assays indicated that an ATF4/c-Jun heterodimer was the responsible transcription factor. RNA interference targeting c-Jun or ATF4 eliminated arsenite-induced APE1 transcription. Suppression of Ape1 or ATF4 sensitized both mouse fibroblasts (10T(1/2)) and human lymphoblastoid cells (TK6) to arsenite cytotoxicity. Expression of Ape1 from a transgene did not efficiently restore arsenite resistance in ATF4-depleted cells but did offset initial accumulation of abasic DNA damage following arsenite treatment. Mutagenesis by arsenite (at the TK and HPRT loci in TK6 cells) was observed only for ATF4-depleted cells, which was strongly offset by Ape1 expression from a transgene. Therefore, the ATF4-mediated up-regulation of Ape1 and other genes plays a key role against arsenite-mediated toxicity and mutagenesis.

Arsenite induces delayed mutagenesis and transformation in human osteosarcoma cells at extremely low concentrations

Arsenite is a human multisite carcinogen, but its mechanism of action is not known. We recently found that extremely low concentrations (</=0.1 microM) of arsenite transform human osteosarcoma TE85 (HOS) cells to anchorage-independence. In contrast to other carcinogens which transform these cells within days of exposure, almost 8 weeks of arsenite exposure are required for transformation. We decided to reexamine the question of arsenite mutagenicity using chronic exposure in a spontaneous mutagenesis assay we previously developed. Arsenite was able to cause a delayed increase in mutagenesis at extremely low concentrations (</=0.1 microM) in a dose-dependent manner. The increase in mutant frequency occurred after almost 20 generations of growth in arsenite. Transformation required more than 30 generations of continuous exposure. We also found that arsenite induced gene amplification of the dihydrofolate reductase (DHFR) gene in a dose-dependent manner. Since HOS cells are able to methylate arsenite at a very low rate, it was possible that active metabolites such as monomethylarsonous acid (MMA(III)) contributed to the delayed mutagenesis and transformation in these cells. However, when the assay was repeated with MMA(III), we found no significant increase in mutagenesis or transformation, suggesting that arsenite-induced delayed mutagenesis and transformation are not caused by arsenite's metabolites, but by arsenite itself. Our results suggest that long-term exposure to low concentrations of arsenite may affect signaling pathways that result in a progressive genomic instability.

Chemical interaction: enhancement and inhibition of clastogenicity

Most environmental exposures involve concurrent or sequential exposure to multiple chemicals in air, water, and food. Interactive effects in carcinogenesis have been described for certain combinations of agents. They are described in terms of enhancement or inhibition of carcinogenesis. Enhancement effects have been documented for cigarette smoking in combination with exposure to asbestos, radon, alcohol, or other exposures. A variety of inhibitors of carcinogenesis have also been described. They are classified into agents preventing formation of carcinogens; blocking agents; and suppressing agents. Assessment of risk from exposure to multiple agents can be derived either from epidemiological studies in relation to actual exposure or from laboratory studies after controlled exposure to different agents. Prediction of how toxic components of mixtures will interact should be based on an understanding of the mechanisms of such interactions. Compounds may interact chemically, yielding new toxic components or causing a change in the biological availability of the existing components or metabolites. In humans, great individual variability in response is to be expected because of genetic heterogeneity or acquired host susceptibility factors. Interaction is thus a key component in the risk assessment process. In this paper, the definition of interaction and the theoretical basis for different types of interaction in cancer causation are reviewed. Epidemiological and experimental studies showing interactive effects of two chemical carcinogens are also presented.

Epidemiological and experimental aspects of metal carcinogenesis: physicochemical properties, kinetics, and the active species

The carcinogenic properties of selected metals and their compounds are reviewed to provide a useful reference for existing knowledge on relationships between physical and chemical forms, kinetics and carcinogenic potential and between epidemiology, bioassays, and short-term tests. Extensive consideration is given to arsenic, beryllium, cadmium, chromium, lead, and nickel. Other metals such as antimony, cobalt, copper, iron, manganese, selenium, and zinc are discussed briefly.

Cobalt exposure and cancer risk

Cobalt is a technically important metal, used mainly as a binder in the hard-metal industry and as a constituent of many alloys. Cobalt compounds are used as drying agents in paints and laquers. Since ancient times, cobalt compounds have been used as coloring agents for pottery, ceramics, and glass. Soluble cobalt salts interfere adversely with cell division, bind irreversibly to nucleic acids in the cell nucleus, induce chromosome aberrations in plants, and are weakly mutagenic in some in vitro tests with cultured animal cells, bacteria, and yeast. Injections or implantation of cobalt metal, cobalt alloys, and cobalt compounds induced local and sometimes metastasizing sarcomas in rats, rabbits, and mice. Mouse is the least susceptible animal. The only published inhalation study with hamsters exposed to CoO aerosols remained non-positive. Indication of possible carcinogenic effects of cobalt alloys or compounds in human populations has arisen from medical use, in hard-metal industries, and at cobalt production. Unfortunately, confounding by nickel and arsenic is a major problem, and the size of most of the investigated populations has been rather small, so none of the investigations alone gives sufficient evidence of a carcinogenic effect in humans, but taken together there is an indication of a carcinogenic potential that should be explored further.

On the mechanism of the comutagenic effect of Cu(II) with ultraviolet light

Although CuCl2 alone is not mutagenic in E. coli or in Chinese hamster cells, exposure of E. coli to CuCl2 during UV-irradiation causes enhancement of UV-mutagenesis. The mechanism for this comutagenic effect appears to be owing to increased DNA damage by the combined treatment of UV and Cu(II) compared with UV or Cu(II) alone. Using a sequencing gel approach, UV alone is found to cause a particular pattern of alkali-labile sites, whereas CuCl2 alone caused few such sites. The combined action of UV + CuCl2 greatly increased the amount of sites over that of UV alone, and caused a change in their pattern. In the presence of high NaCl concentrations, however, Cu(II) is able to induce DNA damage. This latter effect is most likely owing to the formation of hypochlorite ion. The hypothesis that the comutagenic effect of Cu(II) plus UV might be owing to hydroxyl radical formed via a Fenton reaction involving Cu(II) and UV-generated H2O2 was not supported, since no H2O2 is detectable in aqueous medium after UV irradiation, and catalase did not block the DNA damage. These results favor the hypothesis that UV-irradiation of Cu(II) causes a photoactivation, enabling it to generate free radicals, perhaps by reacting with dissolved oxygen.

Detection of genotoxicity of metallic compounds by the bacterial bioluminescence test

Twenty metallic compounds were assayed for their genotoxic mutagenic activity by the bioluminescence test restoration of the luminescence of dark mutant of the luminous bacterium Photobacterium fischeri). The activity of the metals was tested in a liquid medium as well as on a solid medium. K2Cr2O7, MnCl2, BeCl2, KH2AsO4, ZnCl2 and Na2WO4 showed strong activity in liquid medium while AgNO3, Cd(OOCCH3)2, CoCl2, CuCl2, HgCl2, Na2SeO3 and Pb(NO3)2 were more active in the solid medium test. BaCl2, Na2MoO4, NaAsO2, NiSO4, Na2SeO4, RbCl, and SnCl2 were not active in the bioluminescence test. The correlation between the genotoxic activity of the tested metallic compounds in the bioluminescence test and other bacterial tests for genotoxic agents as well as the correlation between these results and the carcinogenicity of these compounds is discussed.

Risk to preimplantation mouse embryos of combinations of heavy metals and radiation

The influence of arsenic, cadmium, lead or mercury on radiation risk to preimplantation mouse embryos in vitro was studied under various conditions. Morphological development, cell proliferation, and formation of micronuclei were used for assessment of risk after combined exposure to these metals and X-rays. No conditions were found under which arsenic altered radiation risk; the effects were merely additive. Cadmium acted similarly, though a few results indicated that morphological development might be impaired more strongly after combined exposure than expected from the addition of the single effects. Lead enhanced radiation risk with regard to micronucleus formation, but had an additive effect only in the case of morphological development and cell proliferation. Of all four metals, mercury had the greatest potential for enhancement of radiation risk, when morphological development and cell proliferation were studied. The observed combination effects exceeded even those effects which were calculated by taking into account the shape of the dose-effect curves (isobologram analysis, envelope of additivity). Mercury neither induced micronuclei nor enhanced their formation in combination experiments.

Cancer risk from inorganics

Inorganic metals and minerals for which there is evidence of carcinogenicity are identified. The risk of cancer from contact with them in the work place, the general environment, and under conditions of clinical (medical) exposure is discussed. The evidence indicates that minerals and metals most often influence cancer development through their action as cocarcinogens. The relationship between the physical form of mineral fibers, smoking and carcinogenic risk is emphasized. Metals are categorized as established (As, Be, Cr, Ni), suspected (Cd, Pb) and possible carcinogens (Table 6), based on the existing in vitro, animal experimental and human epidemiological data. Cancer risk and possible modes of action of elements in each class are discussed. Views on mechanisms that may be responsible for the carcinogenicity of metals are updated and analysed. Some specific examples of cancer risks associated with the clinical use of potentially carcinogenic metals and from radioactive pharmaceuticals used in therapy and diagnosis are presented. Questions are raised as to the effectiveness of conventional dosimetry in accurately measuring risk from radiopharmaceuticals.

DNA-protein crosslinking by heavy metals in Novikoff hepatoma

Crosslinking of proteins to DNA was studied in live intact Novikoff ascites hepatoma cells exposed in vitro to salts of chromium VI, III, and II, nickel II, cadmium II, and to CoCl2, As2O3, and AlK(SO4)2. DNA-protein complexes were separated by high-speed centrifugation of cells solubilized in buffered 4% sodium dodecyl sulfate and assayed by polyacrylamide gel electrophoresis. Hexavalent chromium compounds formed DNA-protein complexes very efficiently. The trivalent, poorly soluble, cupric chromite was nearly as efficient crosslinker as hexavalent Cr, perhaps because phagocytosis facilitated its entry into the cells. The more basic divalent form produced hardly any crosslinks. Most of the crosslinked proteins were common to all of the chromium salts employed. Nickel salts formed DNA-protein crosslinks less efficiently. Most proteins crosslinked by this metal had a high molecular weight ranging from 94,000 to 200,000. There was little qualitative difference between the crosslinked protein patterns for several various nickel (II) salts. Similar results were obtained for cells incubated with cadmium salts. Most of the proteins crosslinked by cadmium had high molecular weights and were similar to those crosslinked by nickel (II). Relatively weak, but significant, crosslinking was also observed when the Novikoff hepatoma cells were exposed to CoCl2, As2O3, or AlK(SO4)2.

Nickel(II) genotoxicity: potentiation of mutagenesis of simple alkylating agents

Many metals have been shown to alter the function of a wide range of enzyme systems, including those involved in DNA repair and replication. To assess the impact in vivo of such metal actions a "Microtitre" fluctuation assay was used to examine the ability of Ni(II) to act as a comutagen with simple alkylating agents. In E. coli, Ni(II) chloride potentiated the mutagenicity of methyl methanesulfonate (MMS) in polymerase-proficient strains (WP2+ and WP2-), but not in polA- strains (WP6 and WP67) or in lexA- (CM561) or recA- (CM571) strains. The absence of UV excision repair (WP2- and WP67) had little, if any, effect. An extended lag phase was seen at 2-4 h in the polA- strains following treatment with Ni(II) chloride and MMS, but normal growth resumed thereafter. Results suggested that mutations induced by MMS were fixed during log phase growth and that more than 2 h of exposure were necessary for potentiation by Ni(II) to be observed. Thus, the extended lag phase probably cannot explain the lack of potentiation. RecA-dependence of the comutagenic effect was corroborated with S. typhimurium TA1535 and TA100. Only in the pKM101 containing strain, TA100, was potentiation of ethyl methanesulfonate (EMS) and MMS by Ni(II) chloride evident. The mucAB genes carried on pKM101 increase the sensitivity of TA100 to a variety of mutagens, providing there is a functional recA gene product. Taken together, the data suggest that Ni(II) acts indirectly, as a comutagen, in bacterial systems, possibly affecting processes involving recA- and/or polA-dependent function(s).

Potassium chromate potentiates frameshift mutagenesis in E. coli and S. typhimurium

Possible comutagenic effects of chromate on frameshift mutagenesis were studied in bacterial assays. In these experiments, cells were treated with potassium chromate and 9-aminoacridine either singly or in combination. Results were analyzed to detect synergistic, additive and antagonistic responses. Data from these investigations show a clear potentiation of 9-aminoacridine-induced mutagenesis in the presence of chromate in S. typhimurium strain TA1537. Results from cell viability assays shows that the effect is not due to a toxicity artifact. Similar results are obtained in E. coli strains 343/358 (repair-proficient parental strain), 343/415 (recA-deficient), and 343/435 (mismatch-repair-deficient). These data indicate the neither induction of recA-protein nor inhibition of mismatch repair is involved in the action of chromate. In E. coli strain 343/447 (DNA polymerase I deficient), the potentiation was observed at lower concentrations of chromate. This finding suggests that polymerase I functions in recovery of cells from 9-aminoacridine-induced DNA damage and that its absence allows some of this damage to be dealt with in a manner which promotes mutagenesis in the presence of chromate. One possible explanation of these findings is that chromate and 9-aminoacridine react chemically to produce a unique mutagen and that damage caused by this mutagen is repaired via some excision process. However, no reaction between chromate and 9-aminoacridine could be detected by TLC under conditions similar to those in the bacterial assays, even at very high concentrations of both agents. Thus, it seems most likely that the potentiation is due to some action of chromate on repair and/or replication at sites of 9-aminoacridine intercalation. Chromate appears, then, to have significant comutagenic actions in bacterial systems.

Metal-induced DNA damage and repair in human diploid fibroblasts and Chinese hamster ovary cells

Cloning efficiency and DNA strand breaks induction were compared in human diploid fibroblasts (HSBP) and chinese hamster ovary (CHO) cells treated with various metal salts. Cadmium (Cd2+), nickel (Ni2+) and chromate (Cr2O7) reduced the cloning efficiency of HSBP cells more than that of CHO cells whereas the reverse was true after treatment with mercury (Hg2+), manganese (Mn2+) and cobalt (Co2+). The effects on cloning efficiency did not consistently correlate with DNA strand breaking activity as all metals except Cr(VI) were more effective at producing DNA strand breaks in CHO cells than in human cells. The differential responses of the two cell types was shown to be only partially due to differences in cellular uptake of metals. DNA breaks induced in human cells by Hg2+ and Cr2O7 were shown most likely to be alkaline labile sites rather than true strand breaks since no damage was detected in a nick translation assay which measures the amount of free 3'-OH terminals. Damage induced by Mn2+ and Co2+, however, appeared to be comprised at least in part by true DNA strand breaks. DNA damage was also induced in HSBP cells following treatment with selenium but only in the presence of reduced glutathione. These studies indicate that DNA damage is not as major a consequence following some metal treatments in human cells as it appears to be in rodent cells. This suggests that rodent models for risk estimation of metal-induced tumorigenesis may not always be appropriate for extrapolation to humans.

The genetic toxicology of metal compounds: II. Enhancement of ultraviolet light-induced mutagenesis in Escherichia coli WP2

Salts of metals which are carcinogenic, noncarcinogenic, or of unknown carcinogenicity were assayed for their abilities to modulate ultraviolet (UV)-induced mutagenesis in Escherichia coli WP2. In addition to the previously reported comutagenic effect of arsenite, salts of three other compounds were found to enhance UV mutagenesis. CuCl2, MnCl2 (and a small effect by KMnO4), and NaMoO4 acted as comutagens in E coli WP2, which has wild-type DNA repair capability, but were much less comutagenic in the repair deficient strain WP2s (uvrA). The survival of irradiated or unirradiated cells was not affected by these compounds. No effects on UV mutagenesis were seen for 16 other metal compounds. We suggest that the comutagenic effects might occur either via metal-induced decreases in the fidelity of repair replication or (in the case of CuCl2) via metal-induced depurination.

Mutagenic effects of some water-soluble metal compounds in a somatic eye-color test system in Drosophila melanogaster

Nickel, cadmium, lead, arsenic, manganese and chromium salts as well as MeHgOH were screened for mutagenicity, using a sensitive somatic eye-color test system in Drosophila melanogaster. The test is based on the insertion of a mobile element which causes instability in the white locus that is somatically enhanced by mutagens. This white locus expression is combined with a mutation, zeste, in another gene, to produce a light yellow eye color. Larval feeding with mutagens causes somatic mutations in the eye imaginal disc cells that develop into easily detectable red spots in the yellow eyes of adult males. Survival tests showed large differences in the toxicity of different metals, but only hexavalent chromium increased the frequency of somatic mutations above the control level. When combined treatments were carried out with MMS and various metals, sodium arsenite caused a reduction of the MMS-induced mutation frequency while methylmercury increased the frequency of somatic spots.

The genetic toxicology of metal compounds: I. Induction of lambda prophage in E coli WP2s(lambda)

A number of metal compounds have been shown to be human carcinogens. Others, while not proven human carcinogens, are able to cause tumors in laboratory animals. Short-term bacterial assays for genotoxic effects have not been successful in predicting the carcinogenicity of metal compounds. We report here the ability of some metal compounds to cause the induction of lambda prophage in E coli WP2s(lambda). By far the strongest inducing ability was observed with K2CrO4, followed by Pb(NO3)2 greater than MnCl2 greater than Ni(OOCCH3)2 greater than CrCl2 greater than NaWO4 greater than Na2MoO4 greater than KMnO4. With the exception of chromate, long-term exposures in a narrow, subtoxic dose range were required in order to demonstrate phage induction. A new microtiter assay for lambda prophage induction, which incorporates these features, is described. This system also was able to detect very small amounts of organic carcinogens.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Dosimetry of genotoxic agents and dose-response relationships of their effects

Dose-response relationships and determination of dose of mutagens and carcinogens are summarized and discussed on the basis of conceptual and kinetic aspects. Different dose definitions may be referred to steps in the chain of events from exposure (or emission) to observed effects. A system is applied to show the influence of various processes on the kinetics of the transfers between consecutive steps. The same system illustrates processes influenced by protraction and fractionation of dose, synergists, comutagens/cocarcinogens, heritable factors, etc. The response at a given dose is expected to depend on the product of consecutive transfer functions. An application of general rules of chemical kinetics shows that when a chemical is introduced at a sufficiently low level, all processes affecting the transfers and therefore the transfer functions themselves become first-order, provided the induction status of enzymes and the cell-division rate remain constant. Under the same conditions, dose-response relationships are expected to be linear, i.e. without "safe" thresholds. However, present knowledge of the kinetics of repair at low levels of DNA damage and of the kinetics of induction of repair functions is not enough complete to be decisive. These considerations and the fact that observed dose-response data in some cases indicate the existence of thresholds but in others appear able to reject the threshold hypothesis lead to the conclusion that, generally, dose-response curves are most probably linear down to dose zero. However, certain mutagens/carcinogens give rise to lesions repaired so effectively that quasi-thresholds appear in certain subpopulations or organs.

Induction of 6-thioguanine-resistant mutants and single-strand scission of DNA by cadmium chloride in cultured Chinese hamster cells

Inducibility of 6-thioguanine-resistant (6TGr) mutants and single-strand scission of DNA by cadmium chloride (CdCl2) was investigated in cultured Chinese hamster V79 cells. Frequency of 6TGr mutants increased concentration dependently by 24-h treatment with CdCl2 up to 3 X 10(-6) M but decreased beyond 3 X 10(-6) M. Mutagenic potency of cadmium in the absence of S9 was about half that of benzo[a]pyrene in the presence of S9 at equitoxic concentrations. Treatment of the cultured cells with cadmium after benzo[a]pyrene treatment was not synergistic but additive to the mutagenicity of benzo[a]pyrene. Single-strand scission of DNA by alkaline elution techniques was observed in the cells treated with CdCl2 for 2 h in a concentration-dependent manner. The single-strand scission by cadmium was detected only in combination with proteinase K digestion of the cell lysates, indicating formation of DNA--protein cross-linking by the metal. These biological and biochemical findings indicate that cadmium is mutagenic in mammalian cells, and its mutagenic effect seems to be accompanied by single-strand scission of DNA.

Metallic elements in fossil fuel combustion products: amounts and form of emissions and evaluation of carcinogenicity and mutagenicity

Metallic elements contained in coal, oil and gasoline are mobilized by combustion processes and may be emitted into the atmosphere, mainly as components of submicron particles. The information about the amounts, composition and form of metal compounds is reviewed for some fuels and combustion processes. Since metal compounds are always contained in urban air pollutants, they have to be considered whenever an evaluation of biological impact of air pollutants is made. The value of currently used bioassays for the evaluation of the role of trace metal compounds, either as major biologically active components or as modifiers of biological effects of organic compounds is assessed. The whole animal bioassays for carcinogenicity do not seem to be an appropriate approach. They are costly, time-consuming and not easily amenable to the testing of complex mixtures. Some problems related to the application and interpretation of short-term bioassays are considered, and the usefulness of such bioassays for the evaluation of trace metal components contained in complex air pollution mixtures is examined.

In vitro assessment of the toxicity of metal compounds : II. Mutagenesis

A review of in vitro mutagenesis assessment of metal compounds in mammalian and nonmammalian test systems has been compiled. Prokaryotic assays are ineffective or inconsistent in their detection of most metals as mutagens, with the notable exception of hexavalent chromium. Mammalian assay systems appear to be similarly inappropriate for the screening of metal compounds based upon the limited number of studies that have employed those compounds having known carcinogenic activity. Although of limited value as screening tests for the detection of potentially carcinogenic metal compounds, the well-characterized in vitro mutagenesis systems may prove to be of significant value as a means to elucidate mechanisms of metal genotoxicity.

Relationship between metal toxicity to subcellular systems and the carcinogenic response

The effects of metals on subcellular organelle functions have been reviewed in relation to carcinogenesis. Perturbations of the normal uptake and metabolism of carcinogens can arise through changes in microsomal enzyme activities, membrane permeabilities, and cell turnover. Metal effects on heme-dependent oxidative functions are well documented and are primarily manifested by increased heme degradation rates (microsomal heme oxygenase activity), decreased heme production (mitochondrial and cytosolic heme biosynthetic enzymes) and, in the case of a few metals, through nuclear effects of metals on the induction of microsomal enzymes. Many metals are accumulated by lysosomes, but known effects of metals on the function of these organelles in sequestering and storing organic compounds are few. Studies of changes in plasma or mitochondrial membrane permeabilities by metals have centered mainly on the susceptibility of membrane ATPase activities to metal ion alteration and on the involvement of metals in lipid peroxidation and free radical formation. Knowledge of the effects of metals on subcellular organelle functions should aid in the understanding of the mechanisms by which metal ions may play a role in the carcinogenic response.