Effects of the genotoxic carcinogen chromium(VI) on basal and hormone-inducible phosphoenolpyruvate carboxykinase gene expression in vivo: correlation with glucocorticoid- and developmentally regulated expression

Integration of proteomic and metabolomic analyses: New insights for mapping informal workers exposed to potentially toxic elements

Occupational exposure to potentially toxic elements (PTEs) is a concerning reality of informal workers engaged in the jewelry production chain that can lead to adverse health effects. In this study, untargeted proteomic and metabolomic analyses were employed to assess the impact of these exposures on informal workers' exposome in Limeira city, São Paulo state, Brazil. PTE levels (Cr, Mn, Ni, Cu, Zn, As, Cd, Sn, Sb, Hg, and Pb) were determined in blood, proteomic analyses were performed for saliva samples (n = 26), and metabolomic analyses in plasma (n = 145) using ultra-high performance liquid chromatography (UHPLC) coupled with quadrupole-time-of-flight (Q-TOF) mass spectrometry. Blood PTE levels of workers, controls, and their family members were determined by inductively coupled plasma-mass spectrometry (ICP-MS). High concentration levels of Sn and Cu were detected in welders' blood (p < 0.001). Statistical analyses were performed using MetaboAnalyst 4.0. The results showed that 26 proteins were upregulated, and 14 proteins downregulated on the welder group, and thirty of these proteins were also correlated with blood Pb, Cu, Sb, and Sn blood levels in the welder group (p < 0.05). Using gene ontology analysis of these 40 proteins revealed the biological processes related to the upregulated proteins were translational initiation, SRP-dependent co-translational protein targeting to membrane, and viral transcription. A Metabolome-Wide Association Study (MWAS) was performed to search for associations between blood metabolites and exposure groups. A pathway enrichment analysis of significant features from the MWAS was then conducted with Mummichog. A total of 73 metabolomic compounds and 40 proteins up or down-regulated in welders were used to perform a multi-omics analysis, disclosing seven metabolic pathways potentially disturbed by the informal work: valine leucine and isoleucine biosynthesis, valine leucine and isoleucine degradation, arginine and proline metabolism, ABC transporters, central carbon metabolism in cancer, arachidonic acid metabolism and cysteine and methionine metabolism. The majority of the proteins found to be statistically up or downregulated in welders also correlated with at least one blood PTE level, providing insights into the biological responses to PTE exposures in the informal work exposure scenario. These findings shed new light on the effects of occupational activity on workers' exposome, underscoring the harmful effects of PTE.

Mechanisms of chromate carcinogenesis by chromatin alterations

In a dynamic environment, organisms must constantly mount an adaptive response to new environmental conditions in order to survive. Novel patterns of gene expression, driven by attendant changes in chromatin architecture, aid in adaptation and survival. Critical mechanisms in the control of gene transcription govern new spatiotemporal chromatin-chromatin interactions that make regulatory DNA elements accessible to the transcription factors that control the response. Consequently, agents that disrupt chromatin structure are likely to have a direct impact on the transcriptional programs of cells and organisms and to drive alterations in fundamental physiological processes. In this regard, hexavalent chromium (Cr(VI)) is of special interest because it interacts directly with cellular proteins, DNA, and other macromolecules, and is likely to upset cell functions that may cause generalized damage to the organism. Here, we will highlight chromium-mediated mechanisms that disrupt chromatin architecture and discuss how these mechanisms are integral to its carcinogenic properties. Emerging evidence indicates that Cr(VI) targets euchromatin, particularly in genomic locations flanking the binding sites of the essential transcription factors CTCF and AP1, and, in so doing, they disrupt nucleosomal architecture. Ultimately, the ensuing changes, if occurring in critical regulatory domains, may establish a new chromatin state, either toxic or adaptive, that will be governed by the corresponding gene transcription changes in key biological processes associated with that state.

Hexavalent chromium disrupts chromatin architecture

Accessibility of DNA elements and the orchestration of spatiotemporal chromatin-chromatin interactions are critical mechanisms in the regulation of gene transcription. Thus, in an ever-changing milieu, cells mount an adaptive response to environmental stimuli by modulating gene expression that is orchestrated by coordinated changes in chromatin architecture. Correspondingly, agents that alter chromatin structure directly impact transcriptional programs in cells. Heavy metals, including hexavalent chromium (Cr(VI)), are of special interest because of their ability to interact directly with cellular protein, DNA and other macromolecules, resulting in general damage or altered function. In this review we highlight the chromium-mediated mechanisms that promote disruption of chromatin architecture and how these processes are integral to its carcinogenic properties. Emerging evidence shows that Cr(VI) targets nucleosomal architecture in euchromatin, particularly in genomic locations flanking binding sites of the essential transcription factors CTCF and AP1. Ultimately, these changes contribute to an altered chromatin state in critical gene regulatory regions, which disrupts gene transcription in functionally relevant biological processes.

Chromium exposure disrupts chromatin architecture upsetting the mechanisms that regulate transcription

IMPACT STATEMENT

This mini-review highlights current evidence on the mechanisms through which hexavalent chromium (Cr(VI)) disrupts transcriptional regulation, an emerging area of interest and one of the central processes by which chromium induces carcinogenesis. Several studies have shown that Cr(VI) causes widespread DNA damage and disrupts epigenetic signatures, suggesting that chromatin may be a direct Cr(VI) target. The findings discussed here suggest that Cr(VI) disrupts transcriptional regulation by causing genomic architecture changes.

Epithelial-mesenchymal transition during oncogenic transformation induced by hexavalent chromium involves reactive oxygen species-dependent mechanism in lung epithelial cells

Hexavalent chromium [Cr(VI)] is an important human carcinogen associated with pulmonary diseases and lung cancer. Exposure to Cr(VI) induces DNA damage, cell morphological change and malignant transformation in human lung epithelial cells. Despite extensive studies, the molecular mechanisms remain elusive, it is also not known if Cr(VI)-induced transformation might accompany with invasive properties to facilitate metastasis. We aimed to study Cr(VI)-induced epithelial-mesenchymal transition (EMT) and invasion during oncogenic transformation in lung epithelial cells. The results showed that Cr(VI) at low doses represses E-cadherin mRNA and protein expression, enhances mesenchymal marker vimentin expression and transforms the epithelial cell into fibroblastoid morphology. Cr(VI) also increases cell invasion and promotes colony formation. Further studies indicated that Cr(VI) uses multiple mechanisms to repress E-cadherin expression, including activation of E-cadherin repressors such as Slug, ZEB1, KLF8 and enhancement the binding of HDAC1 in E-cadherin gene promoter, but DNA methylation is not responsible for the loss of E-cadherin. Catalase reduces Cr(VI)-induced E-cadherin and vimentin protein expression, attenuates cell invasion in matrigel and colony formation on soft agar. These results demonstrate that exposure to a common human carcinogen, Cr(VI), induces EMT and invasion during oncogenic transformation in lung epithelial cells and implicate in cancer metastasis and prevention.

Long-term exposure to hexavalent chromium inhibits expression of tumor suppressor genes in cultured cells and in mice

We have used mouse hepatoma cells in culture to study acute, short-term high-dose effects of hexavalent chromium on gene regulation directed by the polycyclic aromatic hydrocarbon benzo[a]pyrene (BaP). We find that the mixture engages three major signaling pathways: (i) activation of detoxification genes; (ii) induction of signal transduction effectors; and (iii) epigenetic modification of chromatin marks. Preliminary results in mice exposed to mixtures of low doses of Cr(VI) plus BaP indicate that all three pathways are likely to be engaged also in long-term effects resulting from exposure to environmentally relevant doses of the mixture that inhibit the expression of tumor suppressor genes. Given the toxicity and carcinogenicity of these mixtures, we expect that a two-way analytical approach, from cells in culture to biological effects in vivo and vice versa, will provide a better understanding of the molecular mechanisms responsible for the biological effects of mixtures. By focusing both the in vivo and the in vitro work into long-term, low-dose, environmentally relevant exposures, we expect to develop much needed information pertinent to the type of diseases found in human populations exposed to mixtures of environmental toxicants.

Ameliorating effect of chromium administration on hepatic glucose metabolism in streptozotocin-induced experimental diabetes

Chromium has been recognized as an essential trace element that plays an important role in carbohydrate metabolism. However, the molecular mechanisms involved in its action are not clear. This study was undertaken to understand the mechanism of chromium action in experimental diabetes. Streptozotocin-induced diabetic animals were administered chromium as chromium picolinate (CrP) at a daily dose of 1 mg/kg body weight for a period of 4 weeks. It was observed that chromium complexed with picolinate was effective in lowering plasma glucose levels as well as was able to alleviate polyphagia, polydipsia, and weight loss in diabetic animals. Administration of chromium was also found to normalize glycogen content in liver of diabetic animals to near control levels. The reduction in plasma glucose levels by chromium was accompanied by increase in activity of glycolytic enzymes (e.g., glucokinase, phosphofructokinase, and pyruvate kinase) and by suppression in activity of gluconeogenic enzymes (e.g., glucose-6-phosphatase and phosphoenolpyruvate carboxykinase) in liver. Hepatic glucose uptake was found to be increased by chromium supplementation as demonstrated by decrease in Km and increase in Vmax values in diabetic animals. Chromium levels were lower in the liver of diabetic rats when compared with that of control rats. A negative correlation was observed between plasma glucose and chromium concentration in patients with diabetes. The data suggests that chromium supplementation as CrP is beneficial in correcting hyperglycemia, implying that the modulation of the glucose metabolism by chromium may be therapeutically beneficial in the treatment of diabetes.

The glucocorticoid receptor: a revisited target for toxins

The hypothalamic-pituitary-adrenal (HPA) axis activation and glucocorticoid responses are critical for survival from a number of bacterial, viral and toxic insults, demonstrated by the fact that removal of the HPA axis or GR blockade enhances mortality rates. Replacement with synthetic glucocorticoids reverses these effects by providing protection against lethal effects. Glucocorticoid resistance/insensitivity is a common problem in the treatment of many diseases. Much research has focused on the molecular mechanism behind this resistance, but an area that has been neglected is the role of infectious agents and toxins. We have recently shown that the anthrax lethal toxin is able to repress glucocorticoid receptor function. Data suggesting that the glucocorticoid receptor may be a target for a variety of toxins is reviewed here. These studies have important implications for glucocorticoid therapy.

Mechanism of DNA-protein cross-linking by chromium

Hexavalent chromium is a known inducer of DNA-protein cross-links (DPCs) that contribute to repression of inducible genes and genotoxicity of this metal. Lymphocytic DPCs have also shown potential utility as biomarkers of human exposure to Cr(VI). Here, we examined the mechanism of DPC formation by Cr(VI) and the impact of its main cellular reducers. In vitro reactions of Cr(VI) with one-electron reducing thiols (glutathione and cysteine) or two-electron donating ascorbate were all efficient at DPC production, indicating a dispensable role of Cr(V). No Cr(VI) reducer was able to generate DPC in the presence of Cr(III)-chelating EDTA or phosphate. A critical role of Cr(III) in DNA-protein linkages was further confirmed by dissociation of Cr(VI)-induced DPC by phosphate. EDTA was very inefficient in DPC dissociation, indicating its poor suitability for testing of Cr(III)-mediated bridging and reversal of complex DPC. Reactions containing only one Cr-modified component (protein or DNA) showed that Cr(III)-DNA adduction was the initial step in DPC formation. Cross-linking proceeded slowly after the rapid formation of Cr-DNA adducts, indicating that protein conjugation was the rate-limiting step in DPC generation. Experiments with depletion of glutathione and restoration of ascorbate levels in human lung A549 cells showed that high cellular reducing capacity promotes DPC yield. Overall, our data provide evidence for a three-step cross-linking mechanism involving (i) reduction of Cr(VI) to Cr(III), (ii) Cr(III)-DNA binding, and (iii) protein capture by DNA-bound Cr(III) generating protein-Cr(III)-DNA cross-links.

Chromium cross-links histone deacetylase 1-DNA methyltransferase 1 complexes to chromatin, inhibiting histone-remodeling marks critical for transcriptional activation

Transcriptional regulation of gene expression requires posttranslational modification of histone proteins, which, in concert with chromatin-remodeling factors, modulate chromatin structure. Exposure to environmental agents may interfere with specific histone modifications and derail normal patterns of gene expression. To test this hypothesis, we coexposed cells to binary mixtures of benzo[a]pyrene (B[a]P), an environmental procarcinogen that activates Cyp1a1 transcriptional responses mediated by the aryl hydrocarbon receptor (AHR), and chromium, a carcinogenic heavy metal that represses B[a]P-inducible AHR-mediated gene expression. We show that chromium cross-links histone deacetylase 1-DNA methyltransferase 1 (HDAC1-DNMT1) complexes to Cyp1a1 promoter chromatin and inhibits histone marks induced by AHR-mediated gene transactivation, including phosphorylation of histone H3 Ser-10, trimethylation of H3 Lys-4, and various acetylation marks in histones H3 and H4. These changes inhibit RNA polymerase II recruitment without affecting the kinetics of AHR DNA binding. HDAC1 and DNMT1 inhibitors or depletion of HDAC1 or DNMT1 with siRNAs blocks chromium-induced transcriptional repression by decreasing the interaction of these proteins with the Cyp1a1 promoter and allowing histone acetylation to proceed. By inhibiting Cyp1a1 expression, chromium stimulates the formation of B[a]P DNA adducts. Epigenetic modification of gene expression patterns may be a key element of the developmental and carcinogenic outcomes of exposure to chromium and to other environmental agents.

Chromium inhibits transcription from polycyclic aromatic hydrocarbon-inducible promoters by blocking the release of histone deacetylase and preventing the binding of p300 to chromatin

Co-contamination with complex mixtures of carcinogenic metals, such as chromium, and polycyclic aromatic hydrocarbons is a common environmental problem with multiple biological consequences. Chromium exposure alters inducible gene expression, forms chromium-DNA adducts and chromium-DNA cross-links, and disrupts transcriptional activator-co-activator complexes. We have shown previously that exposure of mouse hepatoma Hepa-1 cells to chromate inhibits the induction of the Cyp1a1 and Nqo1 genes by dioxin. Here we have tested the hypothesis that chromium blocks gene expression by interfering with the assembly of productive transcriptional complexes at the promoter of inducible genes. To this end, we have studied the effects of chromium on the expression of genes induced by benzo[a]pyrene (B[a]P), another aryl hydrocarbon receptor agonist, and characterized the disruption of Cyp1a1 transcriptional induction by chromium. Gene expression profiling by using high density microarray analysis revealed that the inhibitory effect of chromium on B[a]P-dependent gene induction was generalized, affecting the induction of over 50 different genes involved in a variety of signaling transduction pathways. The inhibitory effect of chromium on Cyp1a1 transcription was found to depend on the presence of promoter-proximal sequences and not on the cis-acting enhancer sequences that bind the aryl hydrocarbon receptor-aryl hydrocarbon receptor nuclear translocator complex. By using transient reporter assays and chromatin immunoprecipitation analyses, we found that chromium prevented the B[a]P-dependent release of HDAC-1 from Cyp1a1 chromatin and blocked p300 recruitment. These results provide a mechanistic explanation for the observation that chromium inhibits inducible but not constitutive gene expression.

Chromate-induced epimutations in mammalian cells

Epigenetic gene silencing by aberrant DNA methylation of gene promoter regions is a nonmutagenic but heritable epigenetic mechanism that may mistakenly cause the silencing of important cancer-related tumor suppressor genes. Using a transgenic, V79-derived, mammalian cell line (G12) that contains a bacterial gpt reporter gene in its DNA, we can study carcinogen-induced gene inactivation by mutagenic as well as epigenetic DNA methylation mechanisms. Whereas numerous carcinogens have previously been shown to be mutagenic in these cells, a few carcinogens, including nickel, diethylstilbestrol, and X-rays, are also capable of silencing the G12 cell gpt transgene by aberrant DNA methylation. Here we report for the first time that carcinogenic potassium chromate salts can also induce aberrant DNA methylation in this system. In contrast insoluble barium chromate produced significant level of mutations in these cells but did not cause DNA methylation changes associated with transgene expression.

Differential effects of mitomycin C and doxorubicin on P-glycoprotein expression

Previous studies have demonstrated that mitomycin C (MMC) and other DNA cross-linking agents can suppress MDR1 (multidrug resistance 1) gene expression and subsequent functional P-glycoprotein (Pgp) expression, whereas doxorubicin and other anthracyclines increase MDR1 gene expression. In the present study, with stably transfected Madin-Darby canine kidney C7 epithelial cells expressing a human Pgp tagged with green fluorescent protein under the proximal human MDR1 gene promoter, we demonstrated that MMC and doxorubicin have differential effects on Pgp expression and function. Doxorubicin caused a progressive increase in the cell-surface expression of Pgp and function. In contrast, MMC initially increased plasma membrane expression and function at a time when total cellular Pgp was constant and Pgp mRNA expression had been shown to be suppressed. This was followed by a rapid and sustained decrease in cell-surface expression at later times, presumably as a consequence of the initial decrease in mRNA expression. These studies imply that there are at least two independent chemosensitive steps that can alter Pgp biogenesis: one at the level of mRNA transcription and the other at the level of Pgp trafficking. Understanding the combined consequences of these two mechanisms might lead to novel chemotherapeutic approaches to overcoming drug resistance in human cancers by altering either Pgp mRNA expression or trafficking to the membrane.

Immunotoxicologic effects of inhaled chromium: role of particle solubility and co-exposure to ozone

Soluble and insoluble hexavalent chromium (Cr6+) agents are concomitantly released with ozone (O3) during welding. Although pulmonary/immunologic implications from exposure to each agent individually have been investigated, the effects from simultaneous exposure, as occurs under actual working conditions, are unclear. To investigate immunomodulatory effects of inhaled Cr6+, F-344 rats were exposed for 5 h/day, 5 days/week for 2 or 4 weeks to atmospheres containing soluble potassium chromate (K2CrO4) or insoluble barium chromate (BaCrO4), each alone at 360 micrograms Cr/m3 or in combination with 0.3 ppm O3. One day after the final exposure, rats were euthanized, their lungs were lavaged, and pulmonary macrophages (PAM) were recovered for assessment of basal and inducible functions. Rats inhaling K2CrO4-containing atmospheres had greater levels of total recoverable cells, neutrophils, and monocytes in bronchopulmonary lavage compared to rats exposed to insoluble Cr6+ atmospheres, O3 alone, or air; these rats also had a reduced percentage of PAM, although total PAM levels remained unaffected. Although Cr exposure-related changes in PAM functionality were evident, any dependence upon Cr solubility was variable. K2CrO4-containing atmospheres modulated PAM-inducible interleukins-1 and -6, and tumor necrosis factor-alpha production to a greater degree than those containing BaCrO4. Conversely, BaCrO4-containing atmospheres affected PAM basal nitric oxide production and interferon-gamma-primed/zymosan-stimulated reactive oxygen intermediate production to a greater extent than did those containing K2CrO4. In none of the PAM assays did co-inhalation of O3 result in a modulation of the effects obtained with either Cr6+ compound itself. The results indicate that, while immunomodulatory effects of inhaled Cr6+ upon PAM are related to particle solubility, the co-inhalation of O3 apparently does not cause further modifications of the metal-induced effects.

Molecular basis for effects of carcinogenic heavy metals on inducible gene expression

Certain forms of the heavy metals arsenic and chromium are considered human carcinogens, although they are believed to act through very different mechanisms. Chromium(VI) is believed to act as a classic and mutagenic agent, and DNA/chromatin appears to be the principal target for its effects. In contrast, arsenic(III) is considered nongenotoxic, but is able to target specific cellular proteins, principally through sulfhydryl interactions. We had previously shown that various genotoxic chemical carcinogens, including chromium (VI), preferentially altered expression of several inducible genes but had little or no effect on constitutive gene expression. We were therefore interested in whether these carcinogenic heavy metals might target specific but distinct sites within cells, leading to alterations in gene expression that might contribute to the carcinogenic process. Arsenic(III) and chromium(VI) each significantly altered both basal and hormone-inducible expression of a model inducible gene, phosphoenolpyruvate carboxykinase (PEPCK), at nonovertly toxic doses in the chick embryo in vivo and rat hepatoma H411E cells in culture. We have recently developed two parallel cell culture approaches for examining the molecular basis for these effects. First, we are examining the effects of heavy metals on expression and activation of specific transcription factors known to be involved in regulation of susceptible inducible genes, and have recently observed significant but different effects of arsenic(III) and chromium(VI) on nuclear transcription factor binding. Second, we have developed cell lines with stably integrated PEPCK promoter-luciferase reporter gene constructs to examine effects of heavy metals on promoter function, and have also recently seen profound effects induced by both chromium(VI) and arsenic(III) in this system. These model systems should enable us to be able to identify the critical cis (DNA) and trans (protein) cellular targets of heavy metal exposure leading to alterations in expression of specific susceptible genes. It is anticipated that such information will provide valuable insight into the mechanistic basis for these effects as well as provide sensitive molecular biomarkers for evaluating human exposure.

Synthesis and structural characterization of the N2G-mitomycin C-N2G interstrand cross-link in a model synthetic 23 base pair oligonucleotide DNA duplex

Mitomycin C (MMC) is a genotoxic cancer chemotherapeutic agent that reacts principally at the N2 position of guanine to form one of two predominant monoadducts, or a G-G interstrand cross-link at CpG sites, or a G-G intrastrand cross-link at GpG sites. Previous studies of MMC adduction have principally used very short duplex oligonucleotides (5-15 bp) or very long native duplex DNAs. We examined the formation and structural features of the MMC CpG interstrand cross-link on a model 23 bp synthetic oligonucleotide duplex having the (upper strand) sequence 5'-ATAAATACGTATTTATTTATAAA-3'. MMC was reacted with the duplex oligonucleotide in the presence of sodium dithionite at ratios of 6 mM dithionite: 1.5 mM MMC:0.03 mM duplex. The yield of cross-link in the reaction was determined to be approximately 4.8% by denaturing gel electrophoresis, which represented approximately 75% of the total bound MMC. The cross-linked DNA was isolated to greater than 97% purity in a single step by high temperature size exclusion column chromatography. Characterization of the purified product confirmed that the complex contained exclusively the N2G-MMC-N2G cross-link at the single central CpG site. CD spectroscopy demonstrated a negative band at approximately 290-320 nm which has previously been shown to be characteristic of the MMC cross-link. The relative intensity of this band compared to those reported for shorter duplexes suggested that the majority of the duplex is in a normal B-DNA helical configuration. Base-specific chemical footprinting techniques also indicated that there were subtle but distinct structural perturbations principally within the central four to six base pairs containing and adjacent to the cross-link.

Preferential effects of the chemotherapeutic DNA crosslinking agent mitomycin C on inducible gene expression in vivo

The immediate effects of a single dose of the chemotherapeutic DNA crosslinking agent, mitomycin C (MMC), on the expression of several constitutive and drug-inducible genes were examined in a simple in vivo system, the 14 day chick embryo. We observed no effect of MMC on the steady-state mRNA expression of the constitutively expressed beta-actin, transferrin, or albumin genes. In contrast, MMC treatment significantly altered both the basal and drug-inducible mRNA expression of two glutethimide-inducible genes, 5-aminolevulinic acid (ALA) synthase and cytochrome P450 CYP2H1. The basal expression of these genes was transiently but significantly increased over a 24 hr period following a single dose of MMC. Conversely, MMC significantly suppressed the glutethimide-inducible expression of these genes when administered 1 to 24 hr prior to the inducing drug. The effects of MMC on both basal and drug-inducible ALA synthase and CYP2H1 mRNA expression were principally a result of changes in the transcription rates of these genes. In contrast, MMC treatment had little or no effect on glutethimide-induced expression of ALA synthase or CYP2H1 when administered 1 hr after the inducing drug, suggesting that a very early event in the induction process represents the target for these MMC effects. Covalent binding studies demonstrated that the effects of MMC on gene expression were closely correlated temporally with formation of [3H]-porfiromycin-DNA adducts. These results support the hypothesis that genotoxic chemicals specifically target their effects to inducible genes in vivo.