Solubilization of the carcinogen nickel subsulfide and its interaction with deoxyribonucleic acid and protein

Concise Review of Nickel Human Health Toxicology and Ecotoxicology

Nickel (Ni) metal and Ni compounds are widely used in applications like stainless steel, alloys, and batteries. Nickel is a naturally occurring element in water, soil, air, and living organisms, and is essential to microorganisms and plants. Thus, human and environmental nickel exposures are ubiquitous. Production and use of nickel and its compounds can, however, result in additional exposures to humans and the environment. Notable human health toxicity effects identified from human and/or animal studies include respiratory cancer, non-cancer toxicity effects following inhalation, dermatitis, and reproductive effects. These effects have thresholds, with indirect genotoxic and epigenetic events underlying the threshold mode of action for nickel carcinogenicity. Differences in human toxicity potencies/potentials of different nickel chemical forms are correlated with the bioavailability of the Ni2+ ion at target sites. Likewise, Ni2+ has been demonstrated to be the toxic chemical species in the environment, and models have been developed that account for the influence of abiotic factors on the bioavailability and toxicity of Ni2+ in different habitats. Emerging issues regarding the toxicity of nickel nanoforms and metal mixtures are briefly discussed. This review is unique in its covering of both human and environmental nickel toxicity data.

Role of oxidative stress, mitochondrial membrane potential, and calcium homeostasis in nickel subsulfide-induced human lymphocyte death in vitro

When isolated human lymphocytes were treated in vitro either with various concentrations (0-2 mM) of soluble form of nickel subsulfide (Ni3S2) at 37 degrees C for 4 h or at various times (0-240 min), both concentration- and time-dependent effects of Ni3S2 on lymphocyte death were observed. Increased generation of hydrogen peroxide (H2O2), and superoxide anion (O2-), lipid peroxidation and depletion of both nonprotein (NP-) and protein (P-) sulfhydryl (SH) contents were induced by 1 mM Ni3S2. Ni3S2-induced lymphocyte death was significantly prevented by pre-treatment with either catalase (a H2O2 scavenger), or superoxide dismutase (scavenger of O2- radical), or dimethylthiourea/mannitol (hydroxyl radical scavengers), or deferoxamine (iron-chelator), or glutathione/N-acetylcysteine. Co-treatment with cyclosporin A (a mitochondrial membrane potential' inhibitor) inhibited Ni3S2-induced disturbances in mitochondrial membrane potential, and significantly prevented Ni3S2-induced lymphocyte death. Ni3S2-induced lymphocyte death was also significantly prevented by modulating intracellular calcium fluxes using both Ca2+ channel blockers and intracellular Ca2+ antagonists. Thus, the mechanism of soluble Ni3S2-induced activation of lymphocyte death signalling pathways involves increasing generation of different types of oxidative stress, disturbances in mitochondrial membrane potential and cellular calcium homeostasis' destabilization.

Role of oxidative stress and intracellular calcium in nickel carbonate hydroxide-induced sister-chromatid exchange, and alterations in replication index and mitotic index in cultured human peripheral blood lymphocytes

Human peripheral lymphocytes from whole blood cultures were exposed to either soluble form of nickel carbonate hydroxide (NiCH) (0-60 microM), or of nickel subsulfide (Ni(3)S(2)) (0-120 microM), or of nickel oxide (NiO) (0-120 microM), or nickel sulfate (NiSO(4)) (0-120 microM) for a short duration of 2 h. The treatments occurred 46 h after the beginning of the cultures. The cultures were harvested after a total incubation of 72 h, and sister-chromatid exchange (SCE), replication index (RI), and mitotic index (MI) were measured for each nickel compound. The soluble form of NiCH at 30 microM but those of Ni(3)S(2) and NiO at 120 microM produced significant increase in the SCE per cell compared to the control value, whereas NiSO(4) failed to produce any such significant increase. Except NiSO(4), the soluble forms of NiCH, Ni(3)S(2), and NiO produced significant cell-cycle delay (as measured by the inhibition of RI) as well as significant inhibition of the MI at respective similar concentrations as mentioned above. Pretreatment of human blood lymphocytes with catalase (H(2)O(2) scavenger), or superoxide dismutase (superoxide anion scavenger), or dimethylthiourea (hydroxyl radical scavenger), or deferoxamine (iron chelator), or N-acetylcysteine (general antioxidant) inhibited NiCH-induced SCE, and changes in RI and MI. This suggests the participation of oxidative stress involving H(2)O(2), the superoxide anion radical, the hydroxyl radical, and iron in the NiCH-induced genotoxic responses. Cotreatment of NiCH with either verapamil (inhibitor of intracellular calcium ion ([Ca(2+)](i)) movement through plasma membranes), or dantrolene (inhibitor of [Ca(2+)](i) release from sarcoplasmic reticulum), or BAPTA (Ca(2+) chelator) also inhibited the NiCH-induced responses. These results suggest that [Ca(2+)](i) is also implicated in the genotoxicity of NiCH. Overall these data indicate that various types of oxidative stress including iron-mediated oxidative stress involving the Fenton-Haber/Weiss reaction, and alterations in calcium homeostasis are involved in the genetic damage produced by the soluble form of NiCH.

Role of oxidative stress, mitochondrial membrane potential, and calcium homeostasis in human lymphocyte death induced by nickel carbonate hydroxide in vitro

When isolated human lymphocytes were treated in vitro with various concentrations of soluble form of nickel carbonate hydroxide (NiCH) (0-1 mM), at 37 degrees C for 4 h, both concentration- and time-dependent effects of NiCH on lymphocyte death were observed. Increased generation of hydrogen peroxide (H(2)O(2)), superoxide anion (O(2)(-) ), depletion of both no protein (NP-) and protein (P-) sulfhydryl (SH) contents and lipid peroxidation (LPO) were induced by NiCH. Pretreatment of lymphocytes with either catalase (H(2)O(2) scavenger), or deferoxamine (DFO) (iron chelator), or excess glutathione (GSH) (an antioxidant) not only significantly reduced the NiCH-induced generation of H(2)O(2) and LPO, but also increased the NP-SH and P-SH contents initially reduced by NiCH. NiCH-induced generation of excess O(2)(-) but not excess LPO was significantly reduced by pretreatment with superoxide dismutase (SOD). NiCH-induced lymphocyte death was significantly prevented by pre-treatment with either catalase, or dimethylthiourea/mannitol (hydroxyl radical scavengers), or DFO, or excess GSH/N-acetylcysteine. NiCH-induced lymphocyte death was also significantly prevented by pretreatment with excess SOD. Thus, various types of oxidative stresses play an important role in NiCH-induced lymphocyte death. Cotreatment with cyclosporin A (a specific inhibitor of alteration in mitochondrial membrane potential (DeltaPsi(m)) not only inhibited NiCH-induced alteration in DeltaPsi(m), but also significantly prevented Ni-compound-induced lymphocyte death. Furthermore, NiCH-induced destabilization of cellular calcium homeostasis. As such, NiCH-induced lymphocyte death was significantly prevented by modulating intracellular calcium fluxes such as Ca(2+) channel blockers and intracellular Ca(2+) antagonist. Thus, the mechanism of NiCH (soluble form)-induced activation of lymphocyte death signalling pathways involves not only the excess generation of different types of oxidative stress, but also the induction of alteration in DeltaPsi(m) and destabilization of cellular calcium homeostasis as well.

Nickel compound-induced DNA single-strand breaks in chromosomal and nuclear chromatin in human blood lymphocytes in vitro: Role of oxidative stress and intracellular calcium

The effects of nickel sulfate, and soluble forms of nickel carbonate hydroxide (NiCH), nickel subsulfide, and nickel oxide on delayed induction of DNA single-strand breaks (DNA SSBs) in chromosomal and nuclear chromatin of human blood lymphocytes in culture were studied. After 46 h of initial culture in supplemented RPMI-1640 media at 37 degrees C, human whole blood lymphocytes in culture were exposed to low concentrations (0-15 microM) of different nickel (Ni) compounds for 2 h, whereas only RPMI-1640 medium served as control. Immediately after 2 h of such exposure, both control and Ni-treated cells were washed with the same medium and incubated further in fresh complete RPMI-1640 culture medium for another 24h. After a total 70 h of incubation, cells were then arrested at metaphase. Two hours later, the induction of DNA SSBs involving both metaphase chromosomal and interphase nuclear chromatin was measured using the method of electron microscopy in situ end-labeling. The metaphase chromosomal chromatin showed significantly higher DNA SSBs (as measured by an increase in immunogold particles per microm2 chromatin) due to 15 microM NiCH and NiO when compared to the corresponding control value. Both NiCH and nickel oxide produced significantly higher induction of DNA SSBs than those of nickel subsulfide and nickel sulfate in chromosomal chromatin. The DNA SSBs in chromosomal chromatin were found to be significantly higher than those in nuclear chromatin due to different Ni compounds. Overall, the genotoxic potency seems to be decreased as follows: NiCH>nickel oxide>or=nickel subsulfide>nickel sulfate. Pretreatment of human blood lymphocytes with either catalase (a H2O2 scavenger), or superoxide dismutase (a scavenger of O2- radical) or dimethylthiourea (a hydroxyl radical scavenger), or N-acetylcysteine (GSH precursor) significantly reduced DNA SSBs in both chromosomal and nuclear chromatin induced by NiCH, suggesting the involvement of different types of oxidative stress in such genotoxicity. Deferoxamine (a highly specific iron chelator) pretreatment prevented NiCH-induced DNA SSBs in both chromosomal and nuclear chromatin suggesting a role of iron-mediated oxidative stress generating hydroxyl radical in such genotoxicity. Simultaneous treatment with either verapamil (an inhibitor of Ca 2+ through plasma membranes), or dantrolene (an inhibitor of mobilization of [Ca2+]i from endoplasmic reticulum), or BAPTA (a Ca2+ chelator) significantly reduced Ni compound-induced DNA SSBs in both chromosomal and nuclear chromatin, suggesting that Ni compound-induced destabilization of calcium homeostasis may also involved in the induction of such DNA SSBs.

Interactions of Ni(II) and Cu(II) ions with the hydrolysis products of the C-terminal -ESHH- motif of histone H2A model peptides. Association of the stability of the complexes formed with the cleavage of the -E-S- bond

We studied the interactions of Ni(II) and Cu(II) ions with the synthetic tetrapeptides SHHK- and SAHK-, which were blocked by amidation making them more realistic models of the hydrolysis peptidic products of the hexapeptides models of H2A histone. A combination of potentiometric and spectroscopic techniques (UV/Vis, CD, NMR and EPR) suggested that at pH > 7 both tetrapeptides coordinated equatorially through the imidazole ring of His in position 3, the N-terminal amino group and the two amide nitrogens existing between these groups {NH2, 2N-, NIm} forming 4N square-planar complexes. While in the case of the CuH(-1)L complex with SHHK- a possible axial coordination of the imidazole ring of His in position 2 was suggested, in the case of the analogous NiH(-1)L complex a completely different interaction of the same ring with metal ions was observed. As expected these complexes have the same structures with the hydrolysis products produced from the Ni(II)- or Cu(II)-assisted hydrolysis of previously studied hexapeptide models of the C-terminal of histone H2A, due to their predominance at pH > 7.4. In addition, the competition plots presented herein showed that the synthetic tetrapeptides SHHK- and SAHK- have higher affinity towards Ni(II) and Cu(II) ions than the previously studied hexapeptides, suggesting that metal ions remain bound to the peptidic products during the hydrolysis cleavage. Thus, it can be concluded that the stability of Ni(II) or Cu(II) complexes with the synthetic tetrapeptides and consequently with the real hydrolysis peptidic products is the driving force of the hydrolysis reaction of H2A histone blocked hexapeptide models, presented in previous studies.

Modulation of flyash-induced genotoxicity in Vicia faba by vermicomposting

Cytogenetic effects of pre- and postvermicomposted flyash samples were evaluated on the root meristem cells of Vicia faba. Seedlings of V. faba were directly sown in flyash and cow dung-soil mixtures (20%, 40%, 60%, and 80%) and the lateral roots grown in these test mixtures were sampled at 5 days. Negative control was run parallel in cow dung-soil (CS) mixture alone. One set of flyash-cow dung-soil (FCS) mixture was subjected to vermicomposting by introducing Eisenia foetida species of earthworms for 30 days and the cytogenetic effects were reinvestigated through V. faba root meristems. Chemical analysis carried out prior to vermicomposting revealed high concentrations of heavy metals such as Cr, Cu, Pb, Zn, and Ni in FCS samples. CS samples also showed the presence of these metals. Cytogenetic examinations of root meristems exposed to the FCS mixtures showed significant inhibition of mitotic index (MI), induction of chromosome aberrations (CA), and a significantly increased frequency of mitotic aberrations (MA). The increase of the aberrations was dependent on the flyash concentrations. Roots grown in CS samples also showed chromosomal and MAs; however, the percentage was lower than that observed with FCS and also statistically nonsignificant. Cytogenetic analysis of vermicomposted samples of FCS revealed a 15-45% decline in the aberration frequencies whereas chemical analysis showed a 10-50% decline in the metal concentrations, viz. Cr, Cu, Pb, Zn, and Ni, which indicates E. foetida a potential accumulator of heavy metals and the decline in metal concentrations may be the cause of the decrease in aberration frequencies. The present study indicates the genotoxicity potential of flyash and also the feasibility of vermicomposting for cleanup of metal-contaminated soil to mitigate the toxicity/genotoxicity.

Acute waterborne nickel toxicity in the rainbow trout (Oncorhynchus mykiss) occurs by a respiratory rather than ionoregulatory mechanism

The acute mechanism of toxicity of waterborne nickel (Ni) was investigated in the rainbow trout (Oncorhynchus mykiss) in moderately hard ( approximately 140 mg l(-1) as CaCO(3)) Lake Ontario water, where the 96-h LC(50) for juvenile trout (1.5-3.5 g) was 15.3 mg (12.7-19.0, 95% C.L.) dissolved Ni l(-1). No marked impact of Ni exposure on average unidirectional or net fluxes of Na(+), Cl(-), or Ca(2+) was observed in juvenile trout exposed for 48-60 h to 15.6 mg Ni l(-1) as NiSO(4). Furthermore, when adult rainbow trout (200-340 g) were fitted with indwelling dorsal aortic catheters and exposed for 117 h to 11.6 mg Ni l(-1) as NiSO(4), plasma ions (Na(+), Cl(-), Ca(2+), and Mg(2+)) were all well conserved. However, mean arterial oxygen tension dropped gradually to approximately 35% of control values. This drop in P(aO(2)) was accompanied by an acidosis primarily of respiratory origin. P(aCO(2)) rose to more than double control values with a concomitant drop in arterial pH of 0.15 units. Acute respiratory toxicity was further evidenced by a significant increase in hematocrit (Ht), and plasma lactate, and a significant decrease in spleen hemoglobin (Hb). Following 117 h of exposure to 11.6 mg Ni l(-1), the gill, intestine, plasma, kidney, stomach, and heart accumulated Ni significantly, with increases of 60, 34, 28, 11, 8, and 3-fold, respectively. Brain, white muscle, liver, and bile did not significantly accumulate Ni. Plasma Ni exhibited a remarkable linear increase with time to levels approximately 30-fold higher than controls. We conclude that in contrast to most other metals, Ni is primarily a respiratory, rather than an ionoregulatory, toxicant at exposure levels close to the 96-h LC(50). The implications of a waterborne metal as an acute respiratory toxicant (as opposed to ionoregulatory toxicants such as Cu, Ag, Cd, or Zn) with respect to toxicity modeling are discussed.

Induction of oxidative DNA damage by carcinogenic metals

The metal ions carcinogenic to humans are As, Be, Cd, Cr and Ni, and the candidates also include Co, Cu, Fe and Pt. A range of molecular mechanisms was proposed for these metals, reflecting their diverse chemical properties. The oxidative concept in metal carcinogenesis proposes that some complexes of the above metals (Co, Cr, Cu, Fe, Ni) formed in vivo undergo redox cycling, yielding reactive oxygen species and/or high valence metal ions which oxidize DNA. Some of the products of oxidative DNA damage, including 8-oxoguanine and strand breaks, induce mutations, which may lead to neoplastic transformation. The establishment of metal-binding modes in the cell nucleus and of their reactivity is crucial for the understanding of molecular events in metal carcinogenesis. We have proposed the binding sites for Ni(II) and Cu(II) in core histones (H3, H2A) and sperm protamines (HP2) and, using molecular models, provided evidence for the generation of promutagenic oxidative DNA damage by the bound metals.

DNA-protein crosslinks induced by nickel compounds in isolated rat lymphocytes: role of reactive oxygen species and specific amino acids

Isolated rat lymphocytes in salts-glucose medium (pH 7.2) were incubated with nickel chloride, nickel acetate, nickel sulfate, and a soluble form of nickel subsulfide (0-2 mM) at 37 degrees C for 2 h. The soluble form of nickel subsulfide induced a significant increase in DNA-protein crosslinks (DPXLs) (111%) beginning at 0.5 mM and a maximum increase of 700% from that of the control value was reached at a 2 mM concentration, whereas nickel sulfate produced only a 65% increase of such crosslinks at the 2 mM concentration only. No significant reduction in viability of rat lymphocytes (as measured by trypan blue exclusion) due to these nickel compounds was observed at any concentration used. Time-course studies of DPXLs and cellular viability due to 2 mM nickel subsulfide indicate that DPXL formation may not be due in part to cellular necrosis. Coincubation of nickel subsulfide (2 mM) with l-histidine (16 mM), l-cysteine (4 or 8 mM), or l-aspartic acid (24 mM) significantly reduced the DPXLs induced by 2 mM nickel subsulfide. But Mg(2+) even at 24 mM failed to antagonize nickel subsulfide-induced increase in DPXLs. High concentrations of these amino acids significantly decreased the accumulation of Ni(2+) from nickel subsulfide in lymphocytes, suggesting that such reduction of cellular uptake of Ni(2+) by these amino acids is partly responsible for the potent protective effects of these amino acids against such genotoxicity of nickel subsulfide. In vitro exposure of lymphocytes to nickel subsulfide (0-2 mM) increased the formation of reactive oxygen species (ROS) in a concentration-dependent manner. Furthermore, coincubation of 2 mM nickel subsulfide with catalase, dimethylthiourea, mannitol, or vitamin C at 37 degrees C for 2 h resulted in a significant decrease of nickel subsulfide-induced formation of DPXLs, suggesting that nickel subsulfide-induced DPXLs formation in isolated rat lymphocytes is caused by the formation of ROS. The amino acid treatment also abrogated Ni(3)S(2)-induced generation of ROS. Deferoxamine (a highly specific iron chelator) treatment prevented nickel subsulfide-induced DNA-protein crosslink formation, suggesting that Ni(2+)-induced DPXL formation in rat lymphocytes is caused by the induction of Fenton/Haber-Weiss reaction, generating hydroxyl radicals. The potent protective effects of these specific amino acids against nickel subsulfide-induced DPXL formation in isolated rat lymphocytes may be due in part to impaired cellular uptake of Ni(2+), inhibition of the binding of Ni(2+) to deproteinized DNA, and a reduction in reactive oxygen species.

Molecular models in nickel carcinogenesis

Nickel compounds are known human carcinogens, but the exact molecular mechanisms of nickel carcinogenesis are not known. Due to their abundance, histones are likely targets for Ni(II) ions among nuclear macromolecules. This paper reviews our recent studies of peptide and protein models of Ni(II) binding to histones. The results allowed us to propose several mechanisms of Ni(II)-inflicted damage, including nucleobase oxidation and sequence-specific histone hydrolysis. Quantitative estimations of Ni(II) speciation, based on these studies, support the likelihood of Ni(II) binding to histones in vivo, and the protective role of high levels of glutathione. These calculations indicate the importance of histidine in the intracellular Ni(II) speciation.

Interaction of Nickel(II) with histones: in vitro binding of nickel(II) to the core histone tetramer

The absorption spectra of Ni(II) bound to the core histone tetramer, (H3-H4)2, of chicken erythrocytes in 500 mM NaCl + 100 mM phosphate (pH 7.4) were recorded. A charge transfer band was seen at 317 nm, characteristic of a bond between Ni(II) and the sulfur atom of Cys-110 of histone H3. The conditional affinity constants for Ni(II) binding at pH 7.4 for low and high Ni(II) saturation (log Kc = 4.26 +/- 0.02 and 5.26 +/- 0.11 M-1, respectively) were calculated from spectrophotometric titrations with the use of this band. The binding of Ni(II) to (H3-H4)2 is proposed to involve the Cys-110 and His-113 of different H3 molecules within the tetramer. The competition between histones and low-molecular-weight chelators for Ni(II) in the cell nucleus, histidine and glutathione, is discussed on the basis of the above results, indicating that histone H3 is very likely to bind Ni(II) dissolved intracellularly from phagocytosed particulate nickel compounds.

DNA-Protein crosslinks induced by nickel compounds in isolated rat renal cortical cells and its antagonism by specific amino acids and magnesium ion

Suspensions of isolated renal cortical cells in modified Krebs-Henseleit buffer (pH 7.4) were incubated with nickel chloride, nickel acetate, nickel sulfate, and nickel subsulfide (0-2 mM) at 37 degreesC for 2 h. A significant increase (63%) in DNA-protein crosslinks was observed at 2 mM nickel sulfate, whereas nickel subsulfide induced a significant increase in such crosslinks beginning at 0.5 mM concentration and a maximum increase of 200% of the control value reached at 2 mM concentration. No significant reduction in viability of renal cortical cells (as measured by trypan blue exclusion) was observed due to these nickel compounds at any concentration used. In the second series of experiments, coincubation of nickel subsulfide (2 mM) with l-histidine (8 or 16 mM), l-cysteine (4 or 8 mM), or l-aspartic acid (8 or 24 mM) significantly reduced the DNA-protein crosslinks induced by 2 mM nickel subsulfide. Similarly Mg2+ (24 mM), but not Ca2+ (24 mM), was able to antagonize nickel subsulfide-induced increase in DNA-protein crosslinks. High extracellular levels of Mg2+ and these amino acids significantly decreased the accumulation of Ni2+ from nickel subsulfide in renal cortical cells. Furthermore, these amino acids at high concentrations significantly inhibited the binding of Ni2+ from nickel subsulfide to deproteinized DNA from renal cortical cells, whereas such inhibition due to Mg2+ was close to significant (0.1 > p > 0.05). In vitro exposures of renal cortical cells to nickel subsulfide (0-2 mM) increased the formation of reactive oxygen species in concentration-dependent manner. Furthermore, coincubation of 2 mM nickel subsulfide with either catalase, dimethylthiourea, mannitol, or vitamin C at 37 degreesC for 2 h resulted in a significant decrease of nickel subsulfide-induced formation of DNA-protein crosslinks, suggesting that nickel subsulfide-induced DNA-protein crosslink formation in isolated rat renal cortical cells is caused by the formation of reactive oxygen species. The potent protective effects of these specific amino acids and Mg2+ against nickel subsulfide-induced DNA-protein crosslink formation in isolated renal cortical cells are due to reduction of cellular uptake of Ni2+ and inhibition of the binding of Ni2+ to deproteinized DNA.

Detection of genotoxic effects of heavy metal contaminated soils with plant bioassays

Aim of the present study was the development of a bioassay which enables the detection of genotoxic effects of heavy metal contaminated soils. In the first part of the present study, the data base on metal effects in plant bioassays was extended. Four metal salts, namely Cr(VI)O3, Cr(III)Cl3, Ni(II)Cl2 and Sb(III)Cl3 were tested comparatively in MN tests with pollen tetrad cells of Tradescantia clone #4430 and in meristematic root tip cells of Vicia faba. With Cr6+ and Ni2+, clear-cut dose-effects were observed in a range between 0.75 and 10.0 mM, whereas this was not the case with Cr3+ (range tested 1.25-10 mM) and Sb3+ (range 0.30-5.25 mM). In Vicia, negative results were obtained with the four metal salts under all conditions of test. To compare the mutagenic potencies of the metals, the increases of the regression curves (k-values) were calculated, they indicate the number of MN induced per mM in 100 tetrad cells. The corresponding values for Cr6+ and Ni2+ are 0.87 and 1.05, respectively. It appears that the Tradescantia system is in particular sensitive towards those metal species which cause DNA damage in animals and man such as Cr6+, Cd2+, Ni2+, and Zn2+, whereas no clear positive results were obtained with less harmful metal ions such as Cu2+, Cr3+ or Sb3+. In the second part of the study, the mutagenic effects of four metal contaminated soils and two types of standardized leachates (pH 4.0 and pH 7.0) of these soils were tested in Tradescantia and in Vicia. In addition, chemical analyses were carried out to determine the metal concentrations in the soils and in the extracts. Two of the samples contained highly elevated levels of a number of metals (Zn, Pb, Cu, Cd, Sb, As), one soil came from the Central Austrian Alps and contained high As levels only. Direct exposure of the Tradescantia plants in the soils resulted in a drastic increase of the MN frequencies over the background. The lowest effect was seen with the Slovakian soil which contained in particular Sb and As (4.5-fold increase over the background), with the other soils, the induced frequencies were 11-15-fold over the control values. On the contrary, negative results were obtained upon exposure of Tradescantia cuttings in the leachates and upon implantation of germinated Vicia beans in the soils. The results of the present study indicate that Trad-MN assays with direct exposure of intact plants is an appropriate method which enables to detect genotoxic effects of metal contaminated soils in situ. This simple and fast biomonitoring assays might be a valuable supplement to analytical analyses of contaminated soils.

Magnesium reduces nickel inhibition of DNA polymerization

The activities of DNA polymerization and DNA ligation in extract of Chinese hamster ovary cells were both stimulated by MgCl2. DNA polymerization was stimulated by MgCl2 above 0.25 mM, whereas, MgCl2 above 2 mM was required to stimulate DNA ligation. The activity of DNA polymerization maintained a plateau at MgCl2 1-12 mM, whereas DNA ligation reached a maximal activity at MgCl2 6 mM and decreased thereafter. NiCl2 0.1-0.2 mM also had a stimulatory effect on DNA polymerization, but was much less potent than MgCl2. However, nickel ion (Ni2+) had no detectable stimulating effect on the activity of DNA ligation. In the presence of MgCl2, the activities of DNA polymerization and DNA ligation decreased with increasing concentration of NiCl2. Ni2+ inhibition of DNA polymerization was reduced by increasing the concentration of MgCl2, but increasing the concentration of MgCl2 did not reduce Ni2+ inhibition of DNA ligation. Preincubating cell extract with MgCl2 decreased the Ni2+ inhibition of DNA polymerization but not DNA ligation. These results suggest that Ni2+ may compete with magnesium ion (Mg2+) to reduce DNA polymerization, but this mechanism seems not applicable to Ni2+ inhibition of DNA ligation.

Loss of thrombospondin transcriptional activity in nickel-transformed cells

mRNA from normal Chinese hamster embryo (CHE) cells was transcribed to cDNA and subtracted with an excess of mRNA from Chinese hamster embryo cells transformed by nickel compounds. Here we report the recovery of a sequence found to be highly homologous to the mouse thrombospondin 1 gene that was obtained by this subtraction procedure. Since thrombospondin is antiangiogenic, cancer cells expressing high levels of thrombospondin cannot grow in vivo because capillaries will not proliferate to cells secreting thrombospondin. To examine expression of thrombospondin, normal CHE cells were stained with monoclonal antibodies to human thrombospondin. The protein was present abundantly in the cytoplasm of normal cells but at greatly reduced levels in Ni-transformed cells. Analysis of mRNA by Northern (RNA) blot revealed transcripts in normal cells but little thrombospondin mRNA in Ni-transformed cells. Loss of thrombospondin mRNA expression was related to Ni treatment rather than transformation, since Ni-resistant cells also exhibited fewer thrombospondin transcripts than did wild-type cells. Digestion of genomic DNA with various combinations of restriction enzymes revealed thrombospondin gene patterns that were identical in both cell types, suggesting that there were no major deletions or rearrangements of the gene in the nickel-transformed cells. The inactivation of the thrombospondin gene was further investigated by analyzing the promoter activity of this gene linked to a chloramphenicol acetyltransferase (CAT) reporter plasmid that was transfected into normal and Ni-transformed cells. The CAT activity in normal cells was significantly higher than in Ni-transformed cells, suggesting that the promoter region of thrombospondin was less efficiently transcribed in Ni-transformed cells. We studied the consequences of enhanced expression of the retinoblastoma (Rb) gene, a known tumor suppressor gene, on CAT transcription driven by the human thrombospondin promoter. Cotransfection of an expression vector containing the mouse Rb gene greatly enhanced the transcription from the thrombospondin promoter such that the expression was higher in normal cells than in transformed cells.

Loss of thrombospondin transcriptional activity in nickel-transformed cells

mRNA from normal Chinese hamster embryo (CHE) cells was transcribed to cDNA and subtracted with an excess of mRNA from Chinese hamster embryo cells transformed by nickel compounds. Here we report the recovery of a sequence found to be highly homologous to the mouse thrombospondin 1 gene that was obtained by this subtraction procedure. Since thrombospondin is antiangiogenic, cancer cells expressing high levels of thrombospondin cannot grow in vivo because capillaries will not proliferate to cells secreting thrombospondin. To examine expression of thrombospondin, normal CHE cells were stained with monoclonal antibodies to human thrombospondin. The protein was present abundantly in the cytoplasm of normal cells but at greatly reduced levels in Ni-transformed cells. Analysis of mRNA by Northern (RNA) blot revealed transcripts in normal cells but little thrombospondin mRNA in Ni-transformed cells. Loss of thrombospondin mRNA expression was related to Ni treatment rather than transformation, since Ni-resistant cells also exhibited fewer thrombospondin transcripts than did wild-type cells. Digestion of genomic DNA with various combinations of restriction enzymes revealed thrombospondin gene patterns that were identical in both cell types, suggesting that there were no major deletions or rearrangements of the gene in the nickel-transformed cells. The inactivation of the thrombospondin gene was further investigated by analyzing the promoter activity of this gene linked to a chloramphenicol acetyltransferase (CAT) reporter plasmid that was transfected into normal and Ni-transformed cells. The CAT activity in normal cells was significantly higher than in Ni-transformed cells, suggesting that the promoter region of thrombospondin was less efficiently transcribed in Ni-transformed cells. We studied the consequences of enhanced expression of the retinoblastoma (Rb) gene, a known tumor suppressor gene, on CAT transcription driven by the human thrombospondin promoter. Cotransfection of an expression vector containing the mouse Rb gene greatly enhanced the transcription from the thrombospondin promoter such that the expression was higher in normal cells than in transformed cells.

Mutagenic responses of nickel oxides and nickel sulfides in Chinese hamster V79 cell lines at the xanthine-guanine phosphoribosyl transferase locus

Mutagenesis of several insoluble nickel compounds--crystalline nickel sulfide NiS, nickel subsulfide Ni3S2, nickel oxides (black and green) and soluble NiCl2 was studied in three Chinese hamster cell lines--at the hprt gene of the well-defined V79 cell line, and at gpt in two transgenic derivative cell lines G12 and G10. The transgenic cell line G12 responded very strongly to the insoluble Ni compounds, such that the gpt mutagenesis was at least 20 times higher than the spontaneous mutagenesis and in some experiments was even higher. In contrast the response of the G10 cells was much lower--the mutant frequencies only increased 2-3 times over the controls. In V79 cells, NiS and NiO (black) did not induce a mutagenic response at hprt. Soluble NiCl2 also exhibited no mutagenic activity in V79 cells and induced considerably lower activity than the insoluble compounds in the transgenic G12 cells. Following vitamin E pretreatment of G12 cells for 24 h prior to nickel exposure, increased cell survival was observed for several insoluble Ni compounds whereas vitamin E had no effect on NiCl2 cytotoxicity. With vitamin E pretreatment, significantly lower mutagenic responses in G12 cells were also noted for some insoluble Ni compounds, while no such effect was observed for NiCl2.

Metal carcinogenesis: mechanistic implications

Cancer epidemiology has identified several metal compounds as human carcinogens. Recent evidence suggests that carcinogenic metals induce genotoxicity in a multiplicity of ways, either alone or by enhancing the effects of other agents. This review summarizes current information on the genotoxicity of arsenic, chromium, nickel, beryllium and cadmium compounds and their possible roles in carcinogenesis. Each of these metals is distinct in its primary modes of action; yet there are several mechanisms induced by more than one metal, including: the induction of cellular immunity and oxidative stress, the inhibition of DNA metabolism and repair and the formation of DNA- and/or protein-crosslinks.

Ni3S2 uptake by lung cells and its interaction with plasma membranes

The uptake and biological transformation of Ni3S2 was studied on guinea pig alveolar macrophages (GPAM) in primary culture. Two different pathways are observed: (i) phagocytosis of alpha Ni3S2 crystals and subsequent degradation to minute particles, which are recovered bound to the membranes of phagocytic vacuoles and to lysosomal membranes. These degradation products contain sulphur in very reduced quantities, as revealed by energy-dispersive spectrometry (EDS). (ii) Extracellular degradation to regular round particles (0.1-0.2 microns) and irregular minute particles (10-30 nm). Round particles may enter the cell by pinocytosis and are characterized by the loss of sulphur. Minute particles are bound preferentially to cell membranes, to cytoplasmic organelles, such as endoplasmic reticulum, mitochondria and peroxysomes, to liposomes and to the euchromatinic part of nuclei. EDS analyses in these particles revealed the substitution of sulphur by phosphorus. This observation suggests the formation of an Ni-P complex with the phosphate groups of membranous and liposomal phospholipids and of the euchromatinic part of DNA or RNA. Steady-state fluorescence polarization analysis were carried out on GPAM and, for comparative purposes, on human embryonic pulmonary epithelial cells (L132 cell line). In both cell types they revealed a significant increase of membrane fluidity, induced either by desaturation of aliphatic chains or directly by the cleavage of fatty acid chains.

Changes in protein phosphorylation in wild-type and nickel-resistant cells and their involvement in morphological elongation

Treatment of wild-type Balb/c-3T3 cells with NiCl2 or N6,2-O-dibutyl-adenosine 3',5'-monophosphate (Bt2-cAMP) resulted in a high degree and frequency of cellular elongation. Nickel-resistant Balb/c-3T3 cells (B200) treated with Bt2-cAMP elongated at the same exposure concentration as wild-type cells. In contrast, treatment of the nickel-resistant cells with both non-cytotoxic and cytotoxic doses of NiCl2 failed to induce elongation. Nickel-resistant cells had two-thirds of the total protein-phosphorylation activity of wild-type cells. Both cAMP and NiCl2 enhanced phosphorylation of specific proteins in intact wild-type cells as detected by 32p autoradiography of these proteins separated on two-dimensional gels. A nickel-dependent phosphorylation of specific proteins is seen following NiCl2 treatment of wild-type cells but was not observed in B200 cells. In contrast, the pattern of Bt2-cAMP-stimulated protein phosphorylation was quite similar in both wild-type and nickel-resistant cells. Although it is unclear at present how nickel ions affect the cellular protein-phosphorylation system, these results suggested that targets controlling cellular elongation are sensitive to nickel, are altered in nickel-resistant cells and appear to involve protein phosphorylation. Further characterization of these targets may help in understanding the mechanisms of nickel carcinogenesis.

Toxicity and carcinogenicity of nickel compounds

The toxicity and carcinogenicity of nickel compounds are considered in three broad categories: (1) systemic toxicology, (2) molecular toxicology, and (3) carcinogenicity. The systemic toxicity of nickel compounds is examined based upon human and animal studies. The major organs affected are discussed in three categories: (1) kidney, (2) immune system, and (3) other organs. The second area of concentration is molecular toxicology, which will include a discussion of the chemistry of nickel, its binding to small and large molecular weight ligands, and, finally, its cellular effects. The third major area involves a discussion of the carcinogenicity and genotoxicity of nickel compounds. This section focuses on mechanisms, using studies conducted in vivo and in vitro. It also includes a discussion of the assessment of the carcinogenicity of nickel compounds.

Effect of nickel(II) on DNA-protein binding, thymidine incorporation, and sedimentation pattern of chromatin fractions from intact mammalian cells

Nuclear uptake and chromatin binding of nickel(II) was investigated in Chinese hamster ovary (CHO) cells. The cytoplasmic:nuclear ratio of nickel immediately following treatment was 5:1, but by 24 and 48 hours this ratio decreased to 4:1 and 2:1, respectively, indicating that nickel is retained longer in the nucleus than cytoplasmic nickel. Chromatin was fractionated by sonication and centrifugation into fast-sedimenting, magnesium-insoluble, or magnesium-soluble components. The magnesium-insoluble portion bound more nickel ions and retained the metal longer than either the magnesium-soluble or the fast-sedimenting fractions. Treatment of cells with nickel chloride (NiCl2) decreased the amount of DNA in the magnesium-insoluble fraction but increased the amount of DNA in the fast-sedimenting chromatin fraction. The magnesium-insoluble fraction isolated from nickel-treated cells contained approximately ten times more [35-S]-methionine-labeled protein per milligram DNA compared with untreated cells. The magnesium-soluble and the fast-sedimenting fractions isolated from the nickel-treated cells did not exhibit a similar increase in [35-S]-methionine-labeled protein per milligram of DNA. Nickel treatment suppressed [14-C]-thymidine incorporation into total DNA by 30% compared with untreated cells. However, the magnesium-insoluble chromatin fraction from nickel-treated cells had a tenfold to 20-fold increase in thymidine incorporation, while the other chromatin fractions did not exhibit an increase in thymidine incorporation. These findings indicate that nickel induced widespread alterations in chromatin conformation and preferentially interacted with an Mg-insoluble component of chromatin.

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.

Assessment of the uptake and mutagenicity of nickel chloride in salmonella tester strains

NiCl2 was examined for mutagenic activity in a number of Salmonella tester strains. Conditions were established where there was substantial uptake of the metal into the bacterial cells. However, even when the metal ion was apparently taken up, as determined by metal association with cells, there was a lack of mutagenic activity. These results suggest that nickel is unable to induce basepair or frameshift mutations in Salmonella tester strains and are discussed in relationship to the low binding affinity of Ni(II) for DNA.

Antagonism by essential divalent metals and amino acids of nickel(II)-DNA binding in vitro

In vitro binding of nickel(II) to DNA and the effects of divalent essential metals calcium, magnesium, manganese, copper, and zinc, and of amino acids histidine, cysteine, glutamine, arginine, lysine, alanine, and glycine upon that binding were investigated. Samples of 0.156 mg of calf thymus DNA (0.078 mg/ml in 5 mM ammonium acetate buffer, pH = 7.4) were incubated for 1 hr at 24 degrees C with various concentrations of nickel(II)acetate labeled with 63Ni (0.1 to 250 microM) in the absence or presence of 50 microM concentrations of the essential metal acetates, or with 100 microM concentrations of the amino acids. Free and DNA-bound nickel(II) fractions were separated by gel filtration on Sephadex G-25 and quantified by liquid scintillation counting. Scatchard analysis revealed more than two types of nickel(II)-binding sites and a positive cooperativity of binding at the bound-Ni concentrations below 0.35 microM. The high-affinity nickel(II)-binding sites at DNA were identified as phosphate groups. Their binding capacity equalled 0.043 mumol/mg DNA (approx. 1 mol Ni/70 mol of DNA bases). The apparent dissociation constant of nickel(II) from the high-affinity sites was 5.35 microM. Double reciprocal plots showed the essential divalent metals to be competitive antagonists of nickel(II)-binding to the high-affinity sites, ranking Mg(II) greater than or equal to Mn(II) greater than or equal to Ca(II) greater than or equal to Cu(II) = Zn(II). Similarly, the amino acids antagonized nickel binding to DNA with a relative strength of His greater than Gln greater than or equal to His/Cys greater than Arg greater than Cys greater than or equal to Gly = Ala greater than or equal to Lys. The strongest inhibitors of nickel(II)-DNA binding in vitro appear to be magnesium and manganese, i.e., the same metals that are capable of attenuating nickel carcinogenicity in vivo.

Cross-linking of cytokeratins to DNA in vivo by chromium salt and cis-diamminedichloroplatinum(II)

The in vivo cross-linking of cytokeratins to DNA in intact Novikoff ascites hepatoma cells exposed to the chromium salt K2CrO4 and cis-diamminedichloroplatinum(II) (cis-DDP) was studied. Cytokeratin-DNA complexes were obtained by high-speed centrifugation of cells solubilized in buffered 4% sodium dodecyl sulfate. The cytokeratins were identified electrophoretically and immunologically by use of polyclonal and monoclonal antibodies. Time dependence experiments showed that detectable cross-linking occurred after cells were exposed to K2CrO4 for at least 4 h, and the amount of keratin-DNA complexes increased with the incubation time. Each of the three Novikoff ascites hepatoma cytokeratins (p39, p49, and p56) showed a different apparent rate of cross-link formation with DNA. Cytokeratin-DNA complexes were detectable in our system only with K2CrO4 concentrations of 200 microM or greater, and saturation in cross-linking was effected at approximately 2 mM. Higher K2CrO4 concentrations (up to 5 mM) did not produce further significant increases in the amount of cross-linked cytokeratins. Chromium and cis-DDP cross-linked the same cytokeratins at approximately the same ratios; however, both agents cross-linked the major cytokeratins selectively, since not all cytokeratins present in Novikoff hepatoma cell lysates could be cross-linked to DNA. Further evidence of DNA-cytokeratin complexes was obtained by CsCl gradient centrifugation. Our results document the ability of chromate and cis-DDP to produce DNA-cytokeratin cross-links in vivo and show that in live Novikoff hepatoma cells some, but not all, of the components of intermediate filaments are within cross-linking distance of DNA.

DNA-protein cross-links induced by nickel compounds in intact cultured mammalian cells

The carcinogenic activity of crystalline NiS has been attributed to phagocytosis and intracellular dissolution of the particles to yield Ni2+ which is thought to enter the nucleus and damage DNA. In this study the extent and type of DNA damage in Chinese hamster ovary CHO cells treated with various nickel compounds was assessed by alkaline elution. Both insoluble (crystalline NiS) and soluble (NiCl2) nickel compounds induced single strand breaks and DNA protein cross-links. The single strand breaks were repaired relatively quickly but the DNA-protein cross-links were present and still accumulating 24 h after exposure to nickel. Single strand breakage occurred at both non-cytotoxic and cytotoxic concentrations of nickel, however, DNA-protein cross-linking was absent when cells were exposed to toxic nickel levels. The concentration of nickel that induced DNA-protein cross-linking correlated with those metal concentrations that reversibly inhibited cellular replication.

Differentiation of rat myoblasts. Regulation of turnover of ribosomal proteins and their mRNAs

The regulation of ribosomal proteins (r-proteins) and their mRNAs (rp-mRNAs) was studied in the L6 myoblast, a mammalian cell line which can undergo myogenesis. Upon terminal differentiation, the rate of accumulation of mature ribosomes dropped to approximately 25% of the rate found in undifferentiated myoblasts. Despite the drop in the rate of ribosome accumulation and the rate of rRNA synthesis following terminal differentiation, the rate of r-protein synthesis remained constant. The excess r-protein synthesized in myotubes was quickly degraded. The levels of rp-mRNAs were assessed before and after differentiation. Over 90% of the rp-mRNAs were found on polysomes in both myoblasts and myotubes and represented similar fractions of total poly(A)-rich mRNA. The half-lives of the rp-mRNAs averaged approximately 11 h in both myoblasts and myotubes. In vitro nuclear transcription measurements of a representative rp-mRNA (L32 mRNA) revealed that following differentiation, its rate of synthesis relative to the overall transcription rate dropped by approximately 26% in myotubes while the rate of transcription of rRNA dropped by approximately 77%. These results indicate that the coordination of r-protein and rRNA synthesis observed in myoblasts was uncoupled in myotubes at the level of transcription.

In vitro interaction of 63-nickel(II) with chromatin and DNA from rat kidney and liver nuclei

The interaction of nickel(II) with chromatin was studied in vitro and in isolated nuclei from rat liver and kidney. Nickel(II) bound to chromatin, polynucleosomes (DNA + histone octamer protein complex), and to deproteinized DNA both in intact nuclei and in vitro. The amount of nickel(II) bound depended on the concentration of nickel(II), the presence of chromosomal proteins and the binding sites on DNA which provide a stable coordination environment for nickel(II). The binding of nickel(II) to chromatin and to DNA in whole nuclei was much slower than in vitro indicating that assessibility of the DNA binding sites was influenced by the presence of the nuclear membrane, nuclear matrix and nuclear proteins and/or by the condensed nuclear structure of chromatin. Since DNA containing bound nickel(II) was isolated from chromatin, nickel(II) directly interacted with stable binding sites on the DNA molecule in chromatin. Nickel(II) was associated with the histone and non-histone nuclear proteins as well as the DNA in rat liver and kidney chromatin. Nickel(II) was found to bind to calf thymus histones in vitro. Nickel(II)-nuclear protein and -DNA interactions were investigated by gel electrophoretic analysis of in vitro incubation products. Although nickel-histone and nickel-non-histone protein interactions were completely disrupted by the electrophoretic conditions, fluorography revealed the presence of inert nickel(II)-DNA and/or nickel(II)-DNA-protein complexes.

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.

Analysis of metal-induced DNA lesions and DNA-repair replication in mammalian cells

The potency of several metal compounds in causing lesions in DNA either directly or by exposure of intact cultured cells has been examined using the neutral conditions of nucleoid gradient sedimentation. HgCl2 was clearly the most potent inducer of single-strand breakage when added to isolated nucleoids or when nucleoids were prepared from cells treated with this compound. CaCrO4 , however, caused DNA-strand breaks in nucleoids isolated from cells treated with this agent but did not induce DNA strand breaks when added directly to nucleoids. Although less potent than HgCl2, NiCl2 also caused significant single strand breakage in isolated nucleoids or in nucleoids prepared from cells treated with this metal. Since strand breakage of DNA in intact cells may occur secondary to activation of DNA-dependent nucleases during repair replication, CsCl gradient density sedimentation was utilized to examine whether repair processes were induced by exposure of cells to NiCl2, HgCl2 and CaCrO4 . CaCrO4 and NiCl2 induced substantial DNA-repair activity at concentrations and exposure times where DNA lesions could not be detected whereas HgCl2 induced a 10-fold lower level of DNA-repair activity compared to CaCrO4 at optimal concentrations which again were below the concentrations of this metal that produced measurable DNA lesions. Both the induction of DNA-repair activity and DNA-strand breakage by these metals was concentration- and time-dependent. These results demonstrate some unique aspects of the interaction of HgCl2, NiCl2 and CaCrO4 with the DNA of intact cells and point to the possible important correlation of induction of DNA repair to carcinogenesis since nickel and chromate have clearly been implicated as carcinogens and induce considerable repair whereas HgCl2 is not considered a carcinogen and induces the least DNA repair despite its potency in producing DNA lesions.

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.

The interaction of chromium with nucleic acids

Native and denatured calf thymus DNA, and homopolyribonucleotides were compared with respect to chromium and protein binding after an in vitro incubation with rat liver microsomes, NADPH, and chromium (VI) or chromium (III). A significant amount of chromium bound to DNA when chromium (VI) was incubated with the native or the denatured form of DNA in the presence of microsomes and NADPH. For both native and denatured DNA the amount of protein bound to DNA increased with the amount of chromium bound to DNA. Denatured DNA had much higher amounts of chromium and protein bound than native DNA. There was no interaction between chromium(VI) and either form of DNA in the absence of the complete microsomal reducing system. The binding of chromium(III) to native or denatured DNA was small and relatively unaffected by the presence of microsomes and NADPH. The binding of chromium and protein to polyriboadenylic acid (poly(A], polyribocytidylic acid (poly(C], polyriboguanylic acid (poly(G] and polyribouridylic acid (poly(U] was determined after incubation with chromium(VI) in the presence of microsomes and NADPH. The magnitude of chromium and protein binding to the ribopolymers was found to be poly(G) much greater than poly(A) approximately equal to poly(C) approximately equal to poly(U). These results suggest that the metabolism of chromium(VI) is necessary in order for chromium to interact significantly with nucleic acids. The metabolically-produced chromium preferentially binds to the base guanine and results in DNA-protein cross-links. These findings are discussed with respect to the proposed scheme for the carcinogenicity of chromium(VI).

Sequential events in the induction of transformation in cell culture by specific nickel compounds

Crystalline metal sulfides (i.e., Ni3S2, NiS, CoS, CuS) are potent inducers of transformation in Syrian Hamster Embryo cells while the respective amorphous compounds exhibit low transforming activity. Both crystalline and amorphous compounds are relatively water insoluble and when these particulate compounds (2-4 µm) are added to various types of cultured fibroblasts, they exhibit striking differences in their phagocytosis. These differences in uptake were directly related to their surface charge, since crystalline NiS had a negative surface charge (-27 mV, zeta potential) while amorphous NiS had a positively charged surface (+9 mV, zeta potential). Chemical reduction of amorphous NiS with LiA1H4 resulting in acquisition of a negative surface charge, enhanced phagocytosis and caused an incidence of transformation equalling that of crystalline NiS. Following phagocytosis of crystalline NiS, video time lapse microscopy and studies using(63)NiS show that the particles undergo cytoplasmic dissolution that appears to be mediated by lysosomal interaction. Later, the phagocytized NiS particles aggregate in the perinuclear region, their movement by saltation decreases and in some instances vacuoles form around the particles. Particle dissolution products then enter the nucleus and induce strand breaks in the DNA as determined by alkaline sucrose gradient and alkaline elution techniques. Cesium chloride gradient analysis also indicates that phagocytized NiS particles induce repair of DNA. Damage to DNA appears to be a very selective and sensitive action of all metal compounds with potential carcinogenic activity.