In vitro assessment of the toxicity of metal compounds : III. Effects of metals on DNA structure and function in intact cells

Genotoxic Assessment of Some Inorganic Compounds in Desert Pupfish (Cyprinodon macularius) in the Evaporation Pond from a Geothermal Plant

The frequency of micro nucleated erythrocytes in peripheral blood of the desert pupfish (Cyprinodon macularius) from a geothermal effluent pond is determined and compared to organisms kept in an aquarium. The frequency of micronucleated erythrocytes found in pupfish from the geothermal pond is 2.75 (±2.09) and only 0.44 (±0.52) in captivity organisms. Dissolved As in the ponds doubles the 340 µg L^-1 US-EPA acute quality criteria for aquatic life and Hg equals the 1.77 µg L^-1 chronic criteria. The organisms with high MNE also have significantly high Se, As and Hg concentrations in muscle and liver. Compared to international maximum allowable limits for fish consumption, there is 81× enrichment for Se, 6× for As and 5× for Hg. Although Se is not significantly enriched in water, it is likely that its bioaccumulation occurs via feeding of detritus. The desert pupfish has a significant resistance to extreme metal accumulations and to recover under unpolluted conditions.

Interaction of nickel with UV-light in the induction of cytogenetic effects in human peripheral lymphocytes

Chemical interaction is of major concern in the assessment of risk by regulatory agencies. In the present study, treatment of human lymphocytes with NiSO4 (1-100 microM) or UV-light (200, 1000 ergs/mm2) induced micronuclei (MN) in a dose-dependent fashion. Statistical analysis of the interaction factor (IF), showed that combined treatments of Ni(II) (1-100 microM) with UV-light (200, or 1000 ergs/mm2) interacted antagonistically for the induction of MN. Recently we reported that Ni(II) (0.5-10 microM) with UV-light (200 or 1000 ergs/mm2) or Cr(VI) or X-rays interacted antagonistically for the induction of sister chromatid exchanges (SCE), in peripheral human lymphocytes. These observations suggest that nickel present in complex mixtures may reduce the response, even in the presence of strong MN or SCE inducers, and may lead, therefore, to an underestimate of chemical exposure as assessed by these assays. Furthermore, metals affecting certain microsteps in the process of DNA replication or repair (e.g., histones, polymerases, ligases) may have similar antagonistic effects. Further studies are therefore recommended.

Interaction of nickel with mutagens in the induction of sister chromatid exchanges in human lymphocytes

In this study, individual treatments of human lymphocytes with Ni(II) [0.5-25 microM], Cr(VI) [0.65-1.30 microM], UV-light or X-rays induced SCEs in a dose-dependent fashion, and combined treatments of Ni(II) with Cr(VI), UV-light or X-rays interacted antagonistically. Nickel, at environmentally relevant exposure levels, can have the effect in complex mixtures of reducing an otherwise positive SCE response and could lead to underestimating human exposures to certain classes of chemicals or radiation. Furthermore, our data indicate that antagonism may occur when human lymphocytes are exposed simultaneously to Ni(II) and Cr(VI), suggesting an explanation for epidemiological studies reporting conflicting results for cytogenetic effects in lymphocytes of workers exposed to chromium and nickel.

Fish micronuclei for assessing genotoxicity in water

Micronucleus assays with fish have been shown to be useful in vivo techniques for genotoxicity testing, and show potential for in situ monitoring of water quality. In this paper, we review the literature on the clastogenic effects of chemical and physical agents on fish cells, with emphasis on the induction of micronuclei in teleosts. Included in the review is a description of the mechanisms for formation of micronuclei in cells, and a summary of the various techniques that have been used for micronucleus analysis in fish. This review is directed to assisting laboratories in the development of fish genotoxicity assays for water quality monitoring.

Induction of the human growth hormone gene placed under human hsp70 promoter control in mouse cells: a quantitative indicator of metal toxicity

An in vitro test method for general metal toxicity screening was designed, based on the cellular response to stress. The expression of a transfected human growth hormone gene sequence driven by the human heat-shock protein 70 promoter in NIH/3T3 cells was used as marker of noxious contact with metal compounds. Out of a series of 31 metals, 17 were competent for inducing this stress response system. According to the effective concentration and to the intensity of the response, three different clusters of positive compounds emerged and were ranked as strong, intermediate strength and weak inducers. These results correlated well with data from other in vivo and in vitro metal toxicity studies, including LD50 in mice. Apparently the positive/negative compounds also fitted well with data from genotoxicity and carcinogenesis studies on metal salts.

[The toxicological estimation of the heavy metal content (Cd, Hg, Pb) in food for infants and small children]

There are differences between young and adult organisms regarding toxokinetic aspects and clinical manifestations of heavy metal intoxications. Chronically, toxic Cd intake causes a microcytotic hypochromic anemia in young rats at lower exposure levels and after shorter exposure periods than in adult animals. Cd absorption is increased by co-administration of milk and in conjunction with iron deficiency. After long exposure periods toxic Cd concentrations accumulate in the kidney cortex; this process starts very early in life. In 3-year-old children Cd concentrations in the kidney can reach up to one-third of those found in adults. Hg++ and methyl-Hg can cause Hg encephalopathia, and frequently cause mental retardation in adults. Correspondingly, Hg++ accumulation in the brains of suckling rats is approx. 10 times higher than in grown animals. Milk increases the bioavailability of Hg++. In suckling rats Hg is bound to a greater extent to ligands in the erythrocytes. Methyl-Hg concentrations in breast milk reach 5% of those in maternal plasma and that is a severe hazard for breastfed children of exposed mothers. Toxic Pb concentrations can lead to Pb encephalopathia. A high percentage of surviving children have seizures and show signs of mental retardation. Anemia and reduced intelligence scores were recently observed in children after exposure to very low levels of Pb. Pb absorption is increased in children and after co-administration of milk. There are no definite proofs for carcinogenesis or mutagenesis after oral exposure to Cd, Hg, and Pb in man. Heavy metal concentrations were found in the same order of magnitude in commercial infant formulas and in breast milk. When infant formulas are reconstituted with contaminated tap water, however, Pb and Cd concentrations can be much higher. The average heavy metal uptake from such diets exceeds the provisional tolerable weekly intake levels set by the WHO for adults, calculated on the basis of an average food intake and a downscaled body weight. These considerations do not even provide for differences in absorption and distribution or for the increased sensitivity of children to heavy metal exposure. However, dilution effects for essential heavy metals were observed in fast-growing young children; this effect might be extrapolated to toxic metals. These theoretical considerations are compared with epidemiological evidence. A health statistic from Baltimore shows a decline of Pb intoxications in infants. This observation correlates with a simultaneous decline in exposure to Pb which was due, for example, to decreased use of lead dyes in house paints and the abolition of tin cans for infant food.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of copper on mammalian cell components

Both deficiency and excess of copper induce toxic effects on mammalian cell systems in vivo and in vitro. The effects can be related to the affinities of Cu(II) ions for specific cell components. The nucleus is a potential site for temporary Cu storage while primary targets for free Cu(II) ions are the thiol groups which reduce the ions to Cu(I). Cu(II) ions show a high affinity for nucleic acids, binding with DNA both at intrastrand and interstrand levels, possibly through intercalation between GC pairs. The ability to chelate Cu(II) ions is seen to be of the order: purine greater than purine ribonucleotides greater than purine ribonucleoside greater than pyrimidine ribonucleotides. Copper is an integral part of enzyme activation and enters into the molecular structure of several proteins, like ceruloplasmin. Cu(II) ion is a potential mutagenic agent as seen by its property of inducing infidelity in DNA synthesis in vitro. Teratogenic activities of copper have been reported but carcinogenicity is not yet confirmed. Copper is an essential component of chromatin and is known to accumulate preferentially in the heterochromatic regions. External application of higher doses, however, induces both clastogenic effects and spindle disturbances. In certain forms, inorganic copper enhances the clastogenic activity of other agents. The most widely studied human genetic maladies linked with copper metabolism are Menkes' and Wilson's diseases. Several mutations are known which influence Cu homeostasis in mammals. Such mutations in mice have been used extensively for biochemical studies.

Evidence for the presence of mutagenic compounds other than chromium in particles from mild steel welding

A modified Salmonella/microsome liquid culture assay was used to investigate the mutagenicity of the particulate fraction from mild steel welding. Previous reports have implicated compounds of chromium VI as the mutagenic and toxic agents in welding fumes, since only the particles from welding on stainless steel, which contains 15-25% chromium, were mutagenic, whereas particles from welding on mild steel, which contain less than 0.1% chromium, were not mutagenic or toxic. In this investigation, mild steel particles were shown to contain direct-acting and promutagenic compounds that induced frameshift mutations. The mutagenic agents, which were insoluble in sodium phosphate buffer, did not include chromium VI or organic compounds. Further, the expression of mutation appears to require a cell-particle interaction for the release of the mutagenic species from the particles.

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.

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).

Preliminary studies on the differential removal of products formed in the DNA of various rat organs after chronic administration of a low dose of zinc

Metals can bind to various sites on the bases, the phosphate groups and/or sugars in DNA, depending on the physico-chemical characteristics of the metal ion. Up till now most studies concerned with the interaction of metal ions with DNA and polynucleotides have been carried out in vitro. In the present study, 23 ppm Zn2+ was administered chronically to rats in the drinking water for periods up to 1 week, after which the DNA was isolated from liver, kidney, ileum, colon and brain. The DNA was subsequently hydrolysed and the purine bases separated on Sephadex G-10. Three products of metalation were eluted. There were differences in the overall levels of metalation and in the capacity of the different organs to remove the major product of metalation from the DNA: after 7 days the Zn2+ content of this adduct in brain and kidney was 2 and 4 times respectively that of the controls, but in colon and ileum it had returned to control values, despite the continued administration of Zn2+.

Use of mammalian DNA repair-deficient mutants to assess the effects of toxic metal compounds on DNA

Wild-type and repair-deficient cell lines ( EM9 ) of Chinese Hamster Ovary cells were utilized to assess cytotoxic responses towards metals that produce lesions in DNA. Alkaline elution studies indicated that both CaCrO4 and HgCl2 induced single-strand breaks in the DNA. CaCrO4 and HgCl2 treatments of intact Chinese hamster ovary cells also caused the induction of DNA cross links. The mutant cells, which are thought to have a defect in the repair polymerase enzyme and therefore exhibit greater sensitivity towards a variety of agents that produce lesions in the DNA such as X-rays and ultraviolet-light, also displayed a greater sensitivity, compared to wild-type cells, towards the cytotoxic response of HgCl2 and CaCrO4 . For example, the IC50 (concentration producing a 50% growth inhibition) following exposure for 6-hr to CaCrO4 or 1 hr to HgCl2 was 3.4-fold or 1.8- to 3.9-fold greater in wild-type cells compared to repair-deficient cells respectively. Mutant cells compared to wild-type cells were not more sensitive to growth inhibition by agents whose primary site of action was not at the DNA level (i.e. amphotericin B, trifluoroperazine and cycloheximide). The DNA crosslinks induced by exposure to 10 microM CaCrO4 for 6 hr were almost completely repaired in wild-type cells within 24 hr, whereas in similarly exposed mutant cells this lesion was initially more pronounced and was only partially repaired following a 24-hr recovery period in the absence of CaCrO4 . The repair of single-strand breaks induced by CaCrO4 was more rapid and similar in both wild-type and mutant cells. Since Hg(II) inhibits repair of single-strand breaks, we could not study repair of this lesion induced by this agent; however, at very low concentrations (1 microM) binding of 203Hg(II) to DNA was greater in the mutant cells compared to the wild-type cells. Following removal of 203Hg(II) from the media, mutant cells generally retained more 203Hg bound to DNA relative to the total 203Hg(II) present in the cell. These results demonstrate that an important toxic action of CaCrO4 and HgCl2 involves injury to DNA since the concentrations of these metals causing measurable DNA damage were consistent with their respective cytotoxic concentrations and DNA repair-deficient mutants displayed both enhanced cytotoxicity and decreased repair of metal-induced lesions.

In vitro assessment of the toxicity of metal compounds : IV. Disposition of metals in cells: Interactions with membranes, glutathione, metallothionein, and DNA

This review has focused on several parameters related to the delivery of carcinogenic metal compounds to the cell nucleus as a basis for understanding the intermediates formed between metals and cellular components and the effect of these intermediates on DNA structure and function. Emphasis has been placed on metal interactions at the cellular membrane, including lipid peroxidation, metal interactions with glutathione and their relation to membrane injury, and metal effects on the membrane bound enzyme, Na(+)/K(+) ATPase. Metal binding to metallothionein is also considered, particularly as related to transport and utilization of metal ions and to genetic defects in these processes exemplified in Menkes disease. The ability of cadmium to induce the synthesis of metallothionein more strongly than zinc is also discussed in relation to other toxic and carcinogenic metals. The effects of metal ions on purified DNA and RNA polymerase systems are presented with some of the recent studies using biological ligand-metal complexes. This review points out the importance of considering how metals affect in vitro systems when presented as ionic forms or complexed to relevant biological ligands.