Analysis of metal-induced mutations altering the expression or structure of a retroviral gene in a mammalian cell line

Metallothionein-I- and -II-deficient mice display increased susceptibility to cadmium-induced fetal growth restriction

Maternal cadmium exposure induces fetal growth restriction (FGR), but the underlying mechanisms remain largely unknown. The placenta is the main organ known to protect the fetus from environmental toxins such as cadmium. In this study, we examine the role of the two key placental factors in cadmium-induced FGR. The first is placental enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which is known to protect the fetus from exposure to high cortisol levels and subsequently FGR, and the second the cadmium binding/sequestering proteins metallotheionein (MT)-I and -II. Using the MT-I/II(-/-) mouse model, pregnant mice were administered cadmium, following which pups and placentas were collected and examined. MT-I/II(-/-) pups exposed to cadmium were significantly growth restricted, but neither placental weight nor 11β-HSD2 was altered. Although cadmium administration did not result in any visible structural changes in the placenta, increased apoptosis was detected in MT-I/II(-/-) placentas following cadmium exposure, with a significant increase in levels of both p53 and caspase 3 proteins. Additionally, glucose transporter (GLUT1) was significantly reduced in MT-I/II(-/-) placentas of pups exposed to cadmium, whereas zinc transporter (ZnT-1) remained unaltered. Taken together, these results demonstrate that MT-I/II(-/-) mice are more vulnerable to cadmium-induced FGR. The present data also suggest that increased apoptosis and reduced GLUT1 expression in the placenta contribute to the molecular mechanisms underlying cadmium-induced FGR.

A comparative investigation of DNA strand breaks, sister chromatid exchanges and K-ras gene mutations induced by cadmium salts in cultured human cells

Cadmium (Cd) is a toxic heavy metal of continuing occupational and environmental concern with a wide variety of adverse effects. Several studies have shown that cadmium produces DNA strand breaks, DNA-protein cross-links, oxidative DNA damage, chromosomal aberrations, dysregulation of gene expression resulting in enhanced proliferation, depressed apoptosis and/or altered DNA repair. This study was undertaken to investigate the ability of cadmium chloride (CdCl(2)) and cadmium sulphate (CdSO(4)) to induce point mutations in codon 12 of the K-ras protooncogene assessed by polymerase chain reaction-single strand conformation polymorphisms (PCR-SSCP) and RFLP-enriched PCR methods. Also their genotoxic effects were analyzed by the comet assay and sister chromatid exchanges test. The human lung fibroblast cell line MRC-5 was used for the experiments. Sister chromatid exchanges assay (SCEs) frequencies were significantly increased in cells exposed to cadmium salts in relation to controls (p<0.001). Despite the slow increment observed in the three comet parameters considered when cells were treated with cadmium chloride, significant differences between groups were only found in the variable comet moment (CM) (p<0.005). On the other hand, when cells were exposed to cadmium sulphate, the Kruskal-Wallis test showed highly significant differences between groups for migration, tail moment and comet moment parameters (p<0.001). Nevertheless, a null or weak point mutation induction in K-ras protooncogene was detected using polymerase chain reaction-low ionic strength-single strand conformation polymorphisms (PCR-LIS-SSCP) and RFLP-enriched PCR methods when cells were treated with cadmium salts. Thus, inorganic cadmium produces genotoxicity in human lung fibroblast MRC-5 cells, in the absence of significant point mutation of the K-ras gene.

Effect of cadmium on the floor of the mouth on rats during lactation

Cadmium (Cd) present in the air, drinking water and food has the potential to affect the health of people, mainly those who live in highly industrialized regions. Cd affects placental function, may cross the placental barrier and directly modify fetal development. It is also excreted into milk. The body is particularly susceptible to Cd exposure during perinatal period. The effect on rat oral epithelium (floor of the mouth) after continuous exposure to drinking water containing low levels of Cd during lactation was studied. Female rats were supplied with ad libitum drinking water containing 300 mg/l of CdCl2 throughout the whole lactation period. Control animals received a similar volume of water without Cd. Lactating rats (21 day-old) were killed by lethal dose of anesthetic. The heads were retrieved, fixed in "alfac" solution (alcohol, acetic acid and formaldehyde) for 24 h, serially sectioned in frontal plane, at the level of the first molars. The 6 micro m sections were then stained with hematoxylin and eosin. Nuclear epithelium parameters were estimated, as well as cytoplasm and cell volume, nucleus/cytoplasm ratio, numeric and surface densities, and epithelial thickness. Mean body weight was 34.86 g for the control group and 18.56 g for the Cd-treated group. Histologically, the floor of the mouth epithelium was thinner in the treated group, with smaller and more numerous cells. In this experiment, Cd induced epithelial hypotrophy, indicating a direct action in oral mucosa cells, besides retarded development of the pups.

Cadmium-induced apoptosis in mouse liver

Apoptosis is a process of active cell death, distinct from necrosis and characterized by specific morphological and biochemical features. Although the acute hepatotoxic effects of cadmium (Cd) are well described, little is known about the occurrence of apoptosis in Cd toxicity. Therefore, mice were injected with 5-60 mumol/kg i.p. of Cd and their livers were removed 1.5-48 h later and examined by light microscopy. Cd induced both a time- and dose-dependent increase in apoptotic index, severity of necrosis, and mitotic index. Apoptotic index peaked at 9-14 h after Cd administration and then decreased. The time course of apoptotic DNA fragmentation index, monitored by quantification of oligonucleosomal DNA fragments, correlated with the results obtained by histopathological analysis and a commercial in situ apoptotic DNA detection kit. Liver necrosis, as demonstrated by histology and serum alanine aminotransferase and sorbitol dehydrogenase assays, was most severe 14-48 h after Cd injection. Apoptosis was decreasing by 24 h while necrosis persisted. Replacement of liver tissue by blood lakes (peliosis hepatis) was observed after 14 h. The mitotic index increased gradually with time, indicating compensatory liver cell regeneration. There was a progressive increase in the severity of necrosis, apoptotic index, and mitotic index with increasing dose of Cd. These data demonstrate that apoptosis is a major mode of elimination of critically damaged cells in acute Cd hepatotoxicity in the mouse, and it precedes necrosis.

Evaluation of the direct genotoxic potential of cadmium in four different rodent cell lines

Cadmium is a toxic environmental contaminant that is carcinogenic in humans and laboratory animals. Although the mechanism underlying cadmium carcinogenesis has not yet been determined experimental evidence suggests that the stress-inducible, metal-binding proteins, metallothioneins, may mediate organ specificity. In the present study, four different rodent cell lines (Chinese hamster ovary cells, rat L6 myoblast cells, rat Clone 9 liver cells, and rat TRL 1215 liver cells) were exposed to 0, 1, 5, 10, 50, or 100 microM CdCl2 and monitored for evidence of direct DNA damage. A microfiltration assay was used to measure DNA strand breaks and a filter-binding assay was used to measure DNA-protein crosslinks, two lesions that have been associated with cadmium exposure and may mediate genotoxicity of the metal. Although variability in sensitivity to DNA damage was evident between the different cell lines, in all of the cell lines tested, increases in DNA damage were observed only at cadmium doses that completely arrested cell growth. In addition, in three of the four cell lines tested, induction of metallothionein had no substantial protective effect against cadmium-induced cytotoxicity or genotoxicty. While protection against cadmium-induced DNA strand breakage with metallothionein preinduction was observed in the TRL 1215 rat liver cells, metallothionein preinduction did not protect against cadmium-induced DNA-protein crosslinking in that cell line. Taken together, our results support the hypothesis that cadmium is not directly genotoxic.

Branchpoint and polypyrimidine tract mutations mediating the loss and partial recovery of the Moloney murine sarcoma virus MuSVts110 thermosensitive splicing phenotype

Balanced splicing of retroviral RNAs is mediated by weak signals at the 3' splice site (ss) acting in concert with other cis elements. Moloney murine sarcoma virus MuSVts110 shows a similar balance between unspliced and spliced RNAs, differing only in that the splicing of its RNA is, in addition, growth temperature sensitive. We have generated N-nitroso-N-methylurea (NMU)-treated MuSVts110 revertants in which splicing was virtually complete at all temperatures and have investigated the molecular basis of this reversion on the assumption that the findings would reveal cis-acting elements controlling MuSVts110 splicing thermosensitivity. In a representative revertant (NMU-20), we found that complete splicing was conferred by a G-to-A substitution generating a consensus branchpoint (BP) signal (-CCCUGGC- to -CCCUGAC- [termed G(-25)A]) at -25 relative to the 3' ss. Weakening this BP to -CCCGAC- [G(-25)A,U(-27)C] moderately reduced splicing at the permissive temperature and sharply inhibited splicing at the originally nonpermissive temperature, arguing that MuSVts110 splicing thermosensitivity depends on a suboptimal BP-U2 small nuclear RNA interaction. This conclusion was supported by results indicating that lengthening the short MuSVts110 polypyrimidine tract and altering its uridine content doubled splicing efficiency at permissive temperatures and nearly abrogated splicing thermosensitivity. In vitro splicing experiments showed that MuSVts110 G(-25)A RNA intermediates were far more efficiently ligated than RNAs carrying the wild-type BP, the G(-25)A,U (-27)C BP, or the extended polypyrimidine tract. The efficiency of ligation in vitro roughly paralleled splicing efficiency in vivo [G(-25)A BP > extended polypyrimidine tract > G(-25)A,U(-27)C BP > wild-type BP]. These results suggest that MuSVts110 RNA splicing is balanced by cis elements similar to those operating in other retroviruses and, in addition, that its splicing thermosensitivity is a response to the presence of multiple suboptimal splicing signals.

Oxidative mutagens induce intrachromosomal recombination in yeast

Active oxygen species are thought to be involved in the causation of a number of diseases including cancers. We have investigated the effect of 5 oxidative mutagens, methyl viologen (paraquat), mitomycin C, phenylhydrazine, cumene hydroperoxide and hydrogen peroxide, on the frequency of both intrachromosomal recombination and interchromosomal recombination in the yeast Saccharomyces cerevisiae. All of the chemicals significantly increased the frequency of intrachromosomal recombination in a dose-dependent manner. Only hydrogen peroxide increased the frequency of interchromosomal recombination at the doses tested in this study. A role for hydroxyl radical (.OH) in the effect of H2O2 on recombination is indicated by the ability of the radical scavenger dimethyl sulfoxide (DMSO) to significantly inhibit the induction of both intrachromosomal and interchromosomal recombination by H2O2. The results presented here give further support for the suitability of intrachromosomal recombination measurements as a short-term test for the detection of mutagens and carcinogens.

Comparative investigations of the genotoxic effects of metals in the single cells gel (SCG) assay and the sister chromatid exchange (SCE) test

Sodium arsenite (NaAsO2) and cadmium sulphate (CdSO4) were tested for their ability to induce genotoxic effects in the single cell gel (SCG) assay and the sister chromatid exchange (SCE) test in human blood cultures in vitro. Both metals induced DNA damage in white blood cells that was expressed and detected as DNA migration in the SCG assay. Dose dependent effects were seen for cadmium in concentrations from 5 x 10(-4)-5 x 10(-3) M and for arsenic in concentrations from 2 x 10(-4)-1.5 x 10(-3) M. The distribution of DNA migration among cells, a function of dose, revealed that the majority of exposed cells expressed more DNA damage than cells from control cultures and that with increasing length of DNA migration the variability in migration among cells increased as well. Treatment of cells for 2 hr or 24 hr beginning 48 hr after the start of the blood cultures did not increase the SCE frequency in the case of cadmium but caused a small but significant SCE induction with arsenic at the highest concentration. The metal concentrations which could be investigated in the SCE test were much lower due to a strong toxic effect. Metal concentrations which were toxic in the SCE test were without visible effect in the SCG assay. Thus the two endpoints for the determination of genotoxic effects in vitro differed markedly with respect to the detection of genotoxicity induced by metals. These differences and the biological significance of the findings are discussed.

Cadmium-induced DNA strand damage in cultured liver cells: reduction in cadmium genotoxicity following zinc pretreatment

It is well established that zinc can decrease the carcinogenicity and toxicity of cadmium. In some tissues this may be due to the induction of metallothionein (MT). Therefore, in the present investigation, the effect of zinc pretreatment on cadmium-induced DNA strand damage was determined. The alkaline elution technique was used to assess DNA single strand damage (SSD) in cultured cells derived from rat hepatocytes (TRL-1215), a cell line previously shown to have an active MT gene. The ability to detect SSD in TRL-1215 was established following exposure to gamma-irradiation. Exposure to increasing doses of gamma-irradiation (150-600 rad) resulted in a dose-dependent increase in SSD. Exposure of TRL-1215 cells to CdCl2 for 1 hr at 37 degrees C, using concentrations from 5 to 250 microM, failed to induce detectable SSD in these cells; however, exposure to 500 microM CdCl2 resulted in significant SSD. A time-dependent increase in SSD was demonstrated following a 2 hr continuous exposure to 500 microM CdCl2. Pretreatment of cells with 80 microM zinc acetate, 18 hr prior to exposure to 500 microM CdCl2, resulted in markedly reduced SSD when compared to non-pretreated cells. Zinc pretreatment increased the level of MT gene expression as well as MT protein production. The decrease in DNA strand damage associated with cadmium exposure was not due to a decrease in cadmium accumulation by zinc pretreated cells. In fact, cellular cadmium burden was increased over 2-fold following zinc pretreatment. In addition to protection against cadmium genotoxicity, zinc pretreatment also reduced the cytotoxicity associated with a 2-hr, 500 microM cadmium exposure. These data indicate that zinc pretreatment reduces cadmium genotoxicity, possibly through alterations in MT gene expression.

Toxicological principles of metal carcinogenesis with special emphasis on cadmium

Metals are an important and emerging class of carcinogens. At least three metals, specifically nickel, chromium, and arsenic, are confirmed human carcinogens, and several more are suspected to have carcinogenic potential in man. Considering that the list of known human carcinogens of any type is very small, it becomes clear that metals make up a substantial portion of the list. Furthermore, many metals are very potent carcinogens in laboratory animals. Despite this, relatively little attention has been given to the topic of metal carcinogenesis. The reasons for this relative lack of attention are not clear but perhaps are fostered by a perception that, because metals are the simplest of molecules, their mechanism of action must also be simple. This could not be farther from the truth and, although no clear mechanisms have emerged in the area of metal carcinogenesis, it has become apparent that they are anything but simple. Metal carcinogens possess several unique characteristics including a remarkable target site specificity. Detection of the mechanism, or mechanisms, of metal carcinogenesis has, however, proven elusive, in part because of a wide diversity of metallic carcinogenic agents and the intricate nature of metal interactions in biologic systems. The following review explores this broad topic, with special emphasis on toxicological principles including dose-response relationships and potential mechanisms, using cadmium as an example.

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.

Nickel mutagenesis: alteration of the MuSVts110 thermosensitive splicing phenotype by a nickel-induced duplication of the 3' splice site

We investigated DNA damage caused by carcinogenic metals in a murine sarcoma virus (MuSV)-based mutagenicity assay in which mutations targeted to v-mos expression can be selected. Nickel chloride treatment of NRK cells (termed 6m2 cells) infected with MuSVts110, a retrovirus conditionally defective in viral RNA splicing and cell transformation, caused the outgrowth of transformed "revertants" with changes in the MuSVts110 RNA splicing phenotype. Cadmium and chromium treatment of 6m2 cells resulted in the selection of a second class of revertants with what appeared to be frameshift mutations allowing the translation of a readthrough gag-mos protein. In both classes of metal-induced revertants, viral gene expression was distinct from that observed in revertants arising in untreated 6m2 cultures, arguing that metal treatment did not simply enhance the rate of spontaneous reversion. In one representative nickel revertant line the operative nickel-induced mutation affecting MuSVts110 RNA splicing was a duplication of 70 bases surrounding the 3' splice site. The effect of this mutation was to direct splicing to the most downstream of the duplicated 3' sites and concomitantly relax its characteristic thermosensitivity. These data establish the mutagenic potential of nickel and provide the first example of a defined nickel-induced mutation in a mammalian gene.

Regulation of RNA splicing in gag-deficient mutants of Moloney murine sarcoma virus MuSVts110

We investigated whether the MuSVts110 gag gene product (P58gag) can regulate the novel growth temperature dependence of MuSVts110 RNA splicing. MuSVts110 mutants with either frameshifts or deletions in the gag gene were tested for their ability to maintain the MuSVts110 splicing phenotype. Only small decreases in splicing efficiency and no changes in the thermosensitivity of viral RNA splicing were observed in MuSVts110 gag gene frameshift mutants. Deletions within the gag gene, however, variably decreased MuSVts110 splicing efficiency but had no effect on its thermosensitivity. Another class of MuSVts110 splicing mutants generated by treatment of MuSVts110-infected cells with NiCl2 was also examined. In these "nickel revertants," P58gag is made, but splicing of the viral transcript is nearly complete at all growth temperatures. The splicing of "tagged" viral RNA transcribed from a modified MuSVts110 DNA introduced into nickel revertant cells remained thermosensitive, arguing against trans effects of viral gene products on splicing efficiency. These experiments indicated that neither the MuSVts110 P58gag protein nor any other viral gene product acts in trans to regulate MuSVts110 splicing.

Reversion of thermosensitive splicing defect of Moloney murine sarcoma virus ts110 by oversplicing of viral RNA

Moloney murine sarcoma virus ts110 possesses a thermosensitive splicing defect. By continuously growing nonproducer cells at the nonpermissive temperature, a new class of revertant cells, termed 6m3, that had lost the thermosensitive splicing defect was produced, and six distinct clones were selected. These cell clones were transformed at either permissive or restrictive temperatures. Unlike parental 6m2 cells, which contain two virus-specific RNA species of 4.0 and 3.5 kilobases (kb) at temperatures permissive for transformation, the 3.5-kb RNA was the only virus-specific RNA species detected in 6m3 clones. No new v-mos-containing DNA fragment was observed in Southern blot analysis of these cell clones compared with parental 6m2 cells, indicating that the 3.5-kb RNA was a splicing product rather than a direct transcript. Moreover, these cells expressed P85gag-mos but not P58gag at any temperature. The reversion of the phenotype in 6m3 cell clones appears to be the result of a selective loss of the temperature sensitivity of the splicing reaction, without affecting the thermosensitivity of the protein kinase activity. This change also appears to alter the mechanism regulating the efficiency of the genomic RNA-splicing reaction.

Genotoxicity of chromium compounds. A review

This article reviews approximately 700 results reported in the literature with 32 chromium compounds assayed in 130 short-term tests, using different targets and/or genetic end-points. The large majority of the results obtained with Cr(VI) compounds were positive, as a function of Cr(VI) solubility and bioavailability to target cells. On the other hand, Cr(III) compounds, although even more reactive than Cr(VI) with purified nucleic acids, did not induce genotoxic effects in the majority of studies using intact cells. Coupled with the findings of metabolic studies, the large data-base generated in short-term test systems provides useful information for predicting and interpreting the peculiar patterns of Cr(VI) carcinogenicity.

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.

Activation of thermosensitive RNA splicing and production of a heat-labile P85gag-mos kinase by the introduction of a specific deletion in murine sarcoma virus-124 DNA

Murine sarcoma virus ts110 (MuSVts110) is a conditionally transformation-defective MuSV mutant lacking 1,487 bases found in its wild-type parent, MuSV-349 (MuSV-124). Expression of the MuSVts110 v-mos gene product, P85gag-mos, requires splicing of the viral transcript to align the gag and mos genes in frame. However, this splice event is restricted to growth temperatures of 33 degrees C or lower. No splicing of the viral RNA, no production of P85gag-mos, and, hence, no cell transformation is observed at growth temperatures above 33 degrees C. To determine whether thermosensitive splicing is an intrinsic property of To determine whether thermosensitive splicing is an intrinsic property of MuSVts110 RNA specified by the 1,487-base deletion or a result of a cellular defect, we examined an "equivalent" or MuSVts110 DNA (designated ts32 DNA) constructed by combining wild-type MuSV-124 DNA fragments with a synthetic oligonucleotide to yield an otherwise wild-type viral DNA containing the same 1,487-base deletion as authentic MuSVts110. As observed in control cells (6m2 cells) infected with the authentic MuSVts110 virus, NIH 3T3 cells transfected with ts32 DNA appeared morphologically transformed when grown at 33 degrees C, but were converted to a more normal, flattened shape within a few hours of a shift to 39 degrees C. In concert with these morphological changes, both the processing of the ts32 RNA transcripts and the production of ts32 p85gag-mos kinase were found to be optimal at growth temperatures from 28 to 33 degrees C, but dramatically reduced at 37 to 41 degrees C. Like authentic P85gag-mos, the ts32 P85gag-mos kinase activity was rapidly inactivated by brief exposure to 39 degrees C. These results suggested that the MuSVts110 equivalent is functionally indistinguishable from authentic MuSVts110 and that the novel temperature-sensitive splicing of MuSVts110 transcripts is specified by an intrinsic property of the viral RNA.