Cadmium inhibits mismatch repair by blocking the ATPase activity of the MSH2-MSH6 complex

Discovery of ZIP transporters that participate in cadmium damage to testis and kidney

It has been known for decades that cadmium (Cd) must enter the cell to cause damage, but there was no mechanism to explain genetic differences in response to Cd toxicity until 2005. Starting with the mouse Cdm locus associated with differences in Cd-induced testicular necrosis between inbred strains, a 24.6-centiMorgan region on chromosome 3 was reduced ultimately to 880 kb; in this segment is the Slc39a8 gene encoding the ZIP8 Zn(2+)/HCO(3)(-) symporter. In endothelial cells of the testis vasculature, Cd-sensitive mice exhibit high ZIP8 expression, Cd-resistant mice exhibit very low expression. A 168.7-kb bacterial artificial chromosome (BAC) from a 129S6 (Cd-sensitive) BAC library containing the Slc39a8 gene was inserted into the Cd-resistant C57BL/6J genome: Cd treatment produced testicular necrosis in BAC-transgenic BTZIP8-3 mice but not in non-transgenic littermates, thereby proving that the Slc39a8 gene is indeed the Cdm locus. Cd-induced renal failure also occurred in these BTZIP8-3 mice. Immunohistochemistry showed highly expressed ZIP8 protein in the renal proximal tubular epithelial apical surface, suggesting that ZIP8 participates in Cd-induced renal failure. Slc39a14, most closely evolutionarily related to Slc39a8, encodes differentially-spliced products ZIP14A and ZIP14B that display properties similar to ZIP8. ZIP8 in alveolar cells brings environmental Cd into the organism and ZIP14 in intestinal enterocytes carries Cd into the organism and into the hepatocyte. We believe these two transporters function endogenously as Zn(2+)/HCO(3)(-) symporters important in combating inflammation and carrying out other physiological functions; Cd is able to displace the endogenous cation, enter the cell, and produce tissue damage and disease.

A genome-wide screen of genes involved in cadmium tolerance in Schizosaccharomyces pombe

Cadmium is a worldwide environmental toxicant responsible for a range of human diseases including cancer. Cellular injury from cadmium is minimized by stress-responsive detoxification mechanisms. We explored the genetic requirements for cadmium tolerance by individually screening mutants from the fission yeast (Schizosaccharomyces pombe) haploid deletion collection for inhibited growth on agar growth media containing cadmium. Cadmium-sensitive mutants were further tested for sensitivity to oxidative stress (hydrogen peroxide) and osmotic stress (potassium chloride). Of 2649 mutants screened, 237 were sensitive to cadmium, of which 168 were cadmium specific. Most were previously unknown to be involved in cadmium tolerance. The 237 genes represent a number of pathways including sulfate assimilation, phytochelatin synthesis and transport, ubiquinone (Coenzyme Q10) biosynthesis, stress signaling, cell wall biosynthesis and cell morphology, gene expression and chromatin remodeling, vacuole function, and intracellular transport of macromolecules. The ubiquinone biosynthesis mutants are acutely sensitive to cadmium but only mildly sensitive to hydrogen peroxide, indicating that Coenzyme Q10 plays a larger role in cadmium tolerance than just as an antioxidant. These and several other mutants turn yellow when exposed to cadmium, suggesting cadmium sulfide accumulation. This phenotype can potentially be used as a biomarker for cadmium. There is remarkably little overlap with a comparable screen of the Saccharomyces cerevisiae haploid deletion collection, indicating that the two distantly related yeasts utilize significantly different strategies for coping with cadmium stress. These strategies and their relation to cadmium detoxification in humans are discussed.

Inhibition of rat liver and kidney arginase by cadmium ion

Cadmium ion activates arginase from many species of organisms but is an inhibitor of arginase from many other species. The purpose of this study was to investigate the inhibition of rat liver and kidney arginase by cadmium ion. Rat kidney arginase was inhibited by much lower concentrations of cadmium ion than rat liver arginase. Cadmium ion was a mixed noncompetitive inhibitor of both rat liver and kidney arginase. Cadmium ion enhanced the substrate activation of rat kidney arginase while still inhibiting the enzyme. Cadmium ion prevented the substrate inhibition of rat kidney arginase by fluoride while still inhibiting the enzyme. Cadmium ion also inhibited rat kidney arginase in the presence of manganese ion.

DNA mismatch repair: molecular mechanism, cancer, and ageing

DNA mismatch repair (MMR) proteins are ubiquitous players in a diverse array of important cellular functions. In its role in post-replication repair, MMR safeguards the genome correcting base mispairs arising as a result of replication errors. Loss of MMR results in greatly increased rates of spontaneous mutation in organisms ranging from bacteria to humans. Mutations in MMR genes cause hereditary nonpolyposis colorectal cancer, and loss of MMR is associated with a significant fraction of sporadic cancers. Given its prominence in mutation avoidance and its ability to target a range of DNA lesions, MMR has been under investigation in studies of ageing mechanisms. This review summarizes what is known about the molecular details of the MMR pathway and the role of MMR proteins in cancer susceptibility and ageing.

The PMS2 subunit of human MutLalpha contains a metal ion binding domain of the iron-dependent repressor protein family

DNA mismatch repair (MMR) is responsible for correcting replication errors. MutLalpha, one of the main players in MMR, has been recently shown to harbor an endonuclease/metal-binding activity, which is important for its function in vivo. This endonuclease activity has been confined to the C-terminal domain of the hPMS2 subunit of the MutLalpha heterodimer. In this work, we identify a striking sequence-structure similarity of hPMS2 to the metal-binding/dimerization domain of the iron-dependent repressor protein family and present a structural model of the metal-binding domain of MutLalpha. According to our model, this domain of MutLalpha comprises at least three highly conserved sequence motifs, which are also present in most MutL homologs from bacteria that do not rely on the endonuclease activity of MutH for strand discrimination. Furthermore, based on our structural model, we predict that MutLalpha is a zinc ion binding protein and confirm this prediction by way of biochemical analysis of zinc ion binding using the full-length and C-terminal domain of MutLalpha. Finally, we demonstrate that the conserved residues of the metal ion binding domain are crucial for MMR activity of MutLalpha in vitro.

Yeast genes involved in cadmium tolerance: Identification of DNA replication as a target of cadmium toxicity

Cadmium (Cd(2+)) is a ubiquitous environmental pollutant and human carcinogen. The molecular basis of its toxicity remains unclear. Here, to identify the landscape of genes and cell functions involved in cadmium resistance, we have screened the Saccharomyces cerevisiae deletion collection for mutants sensitive to cadmium exposure. Among the 4866 ORFs tested, we identified 73 genes whose inactivation confers increased sensitivity to Cd(2+). Most were previously unknown to play a role in cadmium tolerance and we observed little correlation between the cadmium sensitivity of a gene deletant and the variation in the transcriptional activity of that gene in response to cadmium. These genes encode proteins involved in various functions: intracellular transport, stress response and gene expression. Analysis of the sensitive phenotype of our "Cd(2+)-sensitive mutant collection" to arsenite, cobalt, mercury and H(2)O(2) revealed 17 genes specifically involved in cadmium-induced response. Among them we found RAD27 and subsequently DNA2 which encode for proteins involved in DNA repair and replication. Analysis of the Cd(2+)-sensitivity of RAD27 (rad27-G67S) and DNA2 (dna2-1) separation of function alleles revealed that their activities necessary for Okazaki fragment processing are essential in conditions of cadmium exposure. Consistently, we observed that wild-type cells exposed to cadmium display an enhanced frequency of forward mutations to canavanine resistance and minisatellite destabilisation. Taken together these results provide a global picture of the genetic requirement for cadmium tolerance in yeast and strongly suggest that DNA replication, through the step of Okazaki fragment processing, is a target of cadmium toxicity.

Carmustine-resistant cancer cells are sensitized to temozolomide as a result of enhanced mismatch repair during the development of carmustine resistance

The cytotoxicity of the monofunctional alkylator temozolomide (TMZ) is mediated by mismatch repair (MMR) triggered by O(6)-alkylguanine, whereas MMR protects cells against bifunctional alkylators, including carmustine (BCNU). Therefore, TMZ may be cytotoxic to BCNU-resistant cancer cells because MMR affects sensitivity to TMZ and BCNU in a converse way. We evaluated TMZ cytotoxicity on BCNU-resistant variant (CEM-R) compared with the parental CCRF-CEM cell line (CEM-S). The mechanisms of its BCNU-resistance involved DNA repairs including nucleotide excision repair, base excision repair, alkylguanine alkyltransferase, MMR, and apoptotic and survival pathways. In particular, transcript levels of MMR-related hMLH1 and hMSH2 were enhanced in CEM-R cells. CEM-R cells were 8-fold more BCNU-resistant but surprisingly 9-fold more TMZ-sensitive than were CEM-S cells. Although TMZ-induced adducts include N-alkylated purines and O(6)-alkylguaine, DNA excision repair was enhanced in CEM-R cells, suggesting the efficient repair of N-alkylation adducts. Cotreatment with methoxyamine, a base excision repair inhibitor, did not sensitize CEM-R cells to TMZ, suggesting little or no contribution of N-alkylation to TMZ-induced cytotoxicity. Cotreatment with O(6)-benzylguanine, an alkylguanine alkyltransferase inhibitor, further sensitized CEM-R cells to TMZ, confirming the cytotoxic impact of O(6)-alkylguanine. Cotreatment with cadmium chloride, an MMR inhibitor, disrupted the sensitivity of CEM-R cells to TMZ. The sensitivity to TMZ was reversed in the CEM-R variant clone that had been established by transfecting CEM-R cells with short hairpin hRNA against hMLH1, suggesting the critical role of MMR on sensitization to TMZ. In conclusion, BCNU-resistant CEM-R cells were sensitized to TMZ as a result of enhanced MMR during the development of BCNU resistance.

Vacuolar compartmentation of the cadmium-glutathione complex protects Saccharomyces cerevisiae from mutagenesis

In the yeast Saccharomyces cerevisiae, gamma-glutamyl transferase (gamma-GT; EC 2.3.2.2) is a vacuolar-membrane bound enzyme. In this work we verified that S. cerevisiae cells deficient in gamma-GT absorbed almost 2.5-fold as much cadmium as the wild-type (wt) cells, suggesting that this enzyme might be responsible for the recycle of cadmium-glutathione complex stored in the vacuole. The mutant strain showed difficulty in keeping constant levels of glutathione (GSH) during the stress, although the GSH-reductase activity was practically the same in both wt and mutant strains, before and after metal stress. This difficulty to maintain the GSH levels in the gamma-GT mutant strain led to high levels of lipid peroxidation and carbonyl proteins in response to cadmium, higher than in the wt, but lower than in a mutant deficient in GSH synthesis. Although the increased levels of oxidative stress, gamma-GT mutant strain showed to be tolerant to cadmium and showed similar mutation rates to the wt, indicating that the compartmentation of the GSH-cadmium complex in vacuole protects cells against the mutagenic action of the metal. Confirming this hypothesis, a mutant strain deficient in Ycf1, which present high concentrations of GSH-cadmium in cytoplasm due to its deficiency in transport the complex to vacuole, showed increased mutation rates.

Reaction of bis[bis(tri-tert-butoxysilanethiolato) cadmium(II)] with 3,5-dimethylpyridine - 113Cd NMR solution study

NMR studies on the reaction of bis[bis(tri-tert-butoxysilanethiolato)cadmium(II)] with 3,5-dimethylpyridine in benzene-d(6) and toluene-d(8) solutions provide clear evidence for the equilibrium character of formation of mixed S,N-ligand cadmium complex.

Inactivation of cadmium induced immunotoxicological alterations in rats by Tunisian montmorillonite clay

Cadmium (Cd(2+)) is a heavy metal that is dispersed throughout the modern environment mainly as a result of pollution from a variety of sources. The aims of the current study were to investigate the efficacy of purified Tunisian montmorillonite clay (TMC) to adsorb Cd, to test the stability of the resulting complex under different conditions in vitro, and to utilize the rat bioassay as an in vivo model to evaluate the protective role of TMC against Cd-induced toxicity and immunodysfunction. In the in vitro study, three concentrations of TMC (0.5, 1.0 and 1.5 g/l aqueous solution) and three concentrations of CdCl(2) (25, 50 and 100 ppm) were tested. The results of the in vitro study showed that TMC had a high capacity of adsorbing Cd at different concentrations tested. The adsorption ranged from 95.7-100% of the available CdCl(2) in aqueous solutions. The complex TMC-Cd was stable at different pHs at 37 degrees C. The in vivo results indicated that treatment with CdCl(2) (2.5 mg/kg BW) for 2 weeks resulted in a significant decrease in triglycerides, total protein, creatinine, creatine kinase, immunoglobulin profile (Ig A and Ig G) and T-cell sub-types (CD3(+), CD4(+), CD8(+) and CD56(+)). Whereas, it significantly increase serum level of AST, ALT, LDH and induced degenerative changes in pro-inflammatory cytokines (TNF-alpha and IL-1). Rats treated with TMC alone (400, 600 and 800 mg/kg BW) were comparable to the control regarding all the tested parameters. The combined treatment of CdCl(2) and TMC at the lowest dose (400 mg/kg BW) showed a significant improvement of all tested parameters. It could be concluded that TMC was effective to protect against Cd hazards at a dose as low as 400 mg/kg BW. These results supported our hypothesis that TMC tightly-bind and immobilized Cd resulted in reduction of metal bioavailability in the gastrointestinal tract.

The L1 retrotranspositional stimulation by particulate and soluble cadmium exposure is independent of the generation of DNA breaks

Human exposure to toxic metals is a concern of the highest priority, due to their vast array of biological effects, including carcinogenicity. The particulate (water insoluble) form of several heavy metals presents a higher carcinogenic potential than its soluble counterparts. Our previous work demonstrates that the particulate forms of different heavy metals, such as nickel oxide, cadmium sulfide and mercury sulfide, stimulate human L1 mobile element activity leading to genomic instability. We present data demonstrating that the soluble form of CdCl2 also stimulates L1 retrotransposition in a dose-dependent manner comparable to the insoluble carcinogenic form of this compound. Reproducible results demonstrated a 2 to 3 fold dose-dependent increase in L1 retrotransposition compared to control cells. Heavy metals may cause DNA breaks through the generation of reactive oxygen species. However, evaluation of DNA damage by comet assay revealed no differences between the negative controls and the CdS-treated cells. In addition, active L1 elements express a protein with endonuclease activity that can generate toxicity through the creation of double strand breaks. To determine the contribution of the L1 endonuclease to the toxicity observed in our metal treatment assays, we compared the wildtype L1 vector with an L1 endonuclease-mutant vector. The presence of an active L1 endonuclease did not contribute significantly to the toxicity observed in any of the CdCl2 or CdS doses evaluated. No correlation between the creation of DNA breaks and L1 activity was observed. Alternatively, heavy metals inhibit enzymatic reactions by displacement of cofactors such as Zn and Mg from enzymes. Concomitant treatment with Mg(Ac)2 and Zn(Ac)2 ppb suppresses the stimulatory effect on L1 activity induced by the 3.8 ppb CdS treatment. Overall, these results are consistent with our previous observations, suggesting that the mechanism of L1 stimulation by heavy metals is most likely due to an overall inhibition of DNA repair proteins or other enzymes caused by the displacement of Mg and Zn from cellular proteins.

p53-Dependent but ATM-independent inhibition of DNA synthesis and G2 arrest in cadmium-treated human fibroblasts

This study focused on the activation of cell cycle checkpoint responses in diploid human fibroblasts that were treated with cadmium chloride and the potential roles of ATM and p53 signaling pathways in cadmium-induced responses. The alkaline comet assay indicated that cadmium caused a dose-dependent increase in DNA damage. Cells that were rendered p53-defective by expression of a dominant-negative p53 allele or knockdown of p53 mRNA were more resistant to cadmium-induced inactivation of colony formation than normal and ataxia telangiectasia (AT) cells. Synchronized fibroblasts in S were more sensitive to cadmium toxicity than cells in G1, suggesting that cadmium may target some element of DNA replication. Cadmium produced a dose- and time-dependent inhibition of DNA synthesis. An immediate inhibition was associated with severe delay in progression through S phase and a delayed inhibition seen 24 h after treatment was associated with accumulation of cells in G2. AT and normal cells displayed similar patterns of inhibition of DNA synthesis and G2 delay after treatment with cadmium, while p53-defective cells displayed significantly less of the delayed inhibition of DNA synthesis and accumulation in G2 post-treatment. Total p53 protein and ser15-phosphorylated p53 were induced by cadmium in normal and AT cells. The p53 transactivation target Gadd45alpha was induced in both p53-effective and p53-defective cells after 4 h cadmium treatment, and this was associated with an acute inhibition of mitosis. Cadmium produced a very unusual pattern of toxicity in human fibroblasts, inhibiting DNA replication and inducing p53-dependent growth arrest but without induction of p21(Cip1/Waf1) or activation of Chk1.

Cadmium: cellular effects, modifications of biomolecules, modulation of DNA repair and genotoxic consequences (a review)

Cadmium is an important toxic environmental heavy metal. Occupational and environmental pollution with cadmium results mainly from mining, metallurgy industry and manufactures of nickel-cadmium batteries, pigments and plastic stabilizers. Important sources of human intoxication are cigarette smoke as well as food, water and air contaminations. In humans, cadmium exposures have been associated with cancers of the prostate, lungs and testes. Acute exposures are responsible for damage to these organs. Chronic intoxication is associated with obstructive airway disease, emphysema, irreversible renal failure, bone disorders and immuno-suppression. At the cellular level, cadmium affects proliferation, differentiation and causes apoptosis. It has been classified as a carcinogen by the International Agency for Research on Cancer (IARC). However, it is weakly genotoxic. Indirect effects of cadmium provoke generation of reactive oxygen species (ROS) and DNA damage. Cadmium modulates also gene expression and signal transduction, reduces activities of proteins involved in antioxidant defenses. Several studies have shown that it interferes with DNA repair. The present review focuses on the effects of cadmium in mammalian cells with special emphasis on the induction of damage to DNA, membranes and proteins, the inhibition of different types of DNA repair and the induction of apoptosis. Current data and hypotheses on the mechanisms involved in cadmium genotoxicity and carcinogenesis are outlined.

Mutagenic effect of cadmium on tetranucleotide repeats in human cells

Cadmium is a human carcinogen that affects cell proliferation, apoptosis and DNA repair processes that are all important to carcinogenesis. We previously demonstrated that cadmium inhibits DNA mismatch repair (MMR) in yeast cells and in human cell-free extracts (H.W. Jin, A.B. Clark, R.J.C. Slebos, H. Al-Refai, J.A. Taylor, T.A. Kunkel, M.A. Resnick, D.A. Gordenin, Cadmium is a mutagen that acts by inhibiting mismatch repair, Nat. Genet. 34 (3) (2003) 326-329), but cadmium also inhibits DNA excision repair. For this study, we selected a panel of three hypermutable tetranucleotide markers (MycL1, D7S1482 and DXS981) and studied their suitability as readout for the mutagenic effects of cadmium. We used a clonal derivative of the human fibrosarcoma cell line HT1080 to assess mutation levels in microsatellites after cadmium and/or N-methyl-N-nitro-N-nitrosoguanidine (MNNG) exposure to study effects of cadmium in the presence or absence of base damage. Mutations were measured in clonally expanded cells obtained by limiting dilution after exposure to zero dose, 0.5 microM cadmium, 5 nM MNNG or a combination of 0.5 microM cadmium and 5 nM MNNG. Exposure of HT1080-C1 to cadmium led to statistically significant increases in microsatellite mutations, either with or without concurrent exposure to MNNG. A majority of the observed mutant molecules involved 4-nucleotide shifts consistent with DNA slippage mutations that are normally repaired by MMR. These results provide evidence for the mutagenic effects of low, environmentally relevant levels of cadmium in intact human cells and suggest that inhibition of DNA repair is involved.

Metal inhibition of human N-methylpurine-DNA glycosylase activity in base excision repair

Cadmium (Cd2+), nickel (Ni2+) and cobalt (Co2+) are human and/or animal carcinogens. Zinc (Zn2+) is not categorized as a carcinogen, and rather an essential element to humans. Metals were recently shown to inhibit DNA repair proteins that use metals for their function and/or structure. Here we report that the divalent ions Cd2+, Ni2+, and Zn2+ can inhibit the activity of a recombinant human N-methylpurine-DNA glycosylase (MPG) toward a deoxyoligonucleotide with ethenoadenine (varepsilonA). MPG removes a variety of toxic/mutagenic alkylated bases and does not require metal for its catalytic activity or structural integrity. At concentrations starting from 50 to 1,000 microM, both Cd2+ and Zn2+ showed metal-dependent inhibition of the MPG catalytic activity. Ni2+ also inhibited MPG, but to a lesser extent. Such an effect can be reversed with EDTA addition. In contrast, Co2+ and Mg2+ did not inhibit the MPG activity in the same dose range. Experiments using HeLa cell-free extracts demonstrated similar patterns of inactivation of the varepsilonA excision activity by the same metals. Binding of MPG to the substrate was not significantly affected by Cd2+, Zn2+, and Ni2+ at concentrations that show strong inhibition of the catalytic function, suggesting that the reduced catalytic activity is not due to altered MPG binding affinity to the substrate. Molecular dynamics (MD) simulations with Zn2+ showed that the MPG active site has a potential binding site for Zn2+, formed by several catalytically important and conserved residues. Metal binding to such a site is expected to interfere with the catalytic mechanism of this protein. These data suggest that inhibition of MPG activity may contribute to metal genotoxicity and depressed repair of alkylation damage by metals in vivo.

DNA repair systems as targets of cadmium toxicity

Cadmium (Cd) is a heavy metal and a potent carcinogen implicated in tumor development through occupational and environmental exposure. Recent evidence suggests that proteins participating in the DNA repair systems, especially in excision and mismatch repair, are sensitive targets of Cd toxicity. Cd by interfering and inhibiting these DNA repair processes might contribute to increased risk for tumor formation in humans. In the present review, the information available on the interference of Cd with DNA repair systems and their inhibition is summarized. These actions could possibly explain the indirect contribution of Cd to mutagenic effects and/or carcinogenicity.

The in vitro immune modulation by cadmium depends on the way of cell activation

Among environmental contaminants known for their toxicity and worldwide distribution, heavy metals are of primary concern. Although the toxicology of cadmium (Cd) has been extensively studied, little information is available on the immunomodulation driven by exposure to low doses of Cd. We aimed to evaluate the immunomodulatory effects elicited by short-term exposure of human immunocompetent cells to low biologically relevant doses of Cd in two activation models. Human peripheral blood mononuclear cells, activated either by bacterial antigens (heat-killed Salmonella Enteritidis) or monoclonal antibodies (mAb: anti-CD3/anti-CD28/anti-CD40), were exposed to Cd acetate for 24h. Cell vitality was determined by MTT assay, cytokine release by ELISA, and cytokine gene expression by real-time RT-PCR. The results demonstrated that, in addition to the known toxic effects of Cd, doses from 0.013 to 13.3 microM exert differential effects on cytokine production. In the case of mAb-activation, secretion of interleukin (IL)-1 beta, tumour necrosis factor (TNF)-alpha and interferon (IFN)-gamma was greatly inhibited at low Cd doses compared to production of IL-4 and IL-10. This indicates a type-2-biased immune response. Under stimulation by bacterial antigens, release of IL-10 was highly suppressed compared to that of IFN-gamma and TNF-alpha; IL-4 was undetectable. These results imply that low Cd doses exert immunomodulatory effects and the direction of this modulation depends on the pathway to cell activation. Overall, Cd polarizes the immune response toward type-2 in cells stimulated via T cell receptors. However, a polarized type-1 response induced by bacterial antigens could not be overwhelmed by the effects of Cd.