Differential effects of chromium(VI) on constitutive and inducible gene expression in chick embryo liver in vivo and correlation with chromium(VI)-induced DNA damage

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.

Mechanism of chromium-induced toxicity in lungs, liver, and kidney and their ameliorative agents

Heavy metal Chromium (Cr), can adversely affect humans and their health if accumulated in organs of the body, such as the lungs, liver, and kidneys. Cr (VI) is highly toxic and has a higher solubility in water than Cr (III). One of the most common routes for Cr exposure is through inhalation and is associated with liver, lung, kidney damage, widespread dermatitis, GI tract damage, human lung cancer, cardiomyopathies, and cardiovascular disease. The increase in ROS production has been attributed to most of the damage caused by Cr toxicity. Cr-induced ROS-mediated oxidative stress has been seen to cause a redox imbalance affecting the antioxidant system balance in the body. The Nrf2 pathway dysregulation has been implicated in the same. Deregulation of histone acetylation and methylation has been observed, together with gene methylation in genes such as p16, MGMT, APC, hMLH1, and also miR-143 repression. Several ultra-structural changes have been observed following Cr (VI)-toxicity, including rough ER dilation, alteration in the mitochondrial membrane and nuclear membrane, pycnotic nuclei formation, and cytoplasm vacuolization. A significant change was observed in the metabolism of lipid, glucose, and the metabolism of protein after exposure to Cr. Cr-toxicity also leads to immune system dysregulations with changes seen in the expression of IL-8, IL-4, IgM, lymphocytes, and leukocytes among others. P53, as well as pro-and anti-apoptotic proteins, are involved in apoptosis. These Cr-induced damages can be alleviated via agents that restore antioxidant balance, regulate Nrf-2 levels, or increase anti-apoptotic proteins while decreasing pro-apoptotic proteins.

Synthesis of caffeic acid sulfonamide derivatives and their protective effect against H2O2 induced oxidative damage in A549 cells

Exogenous antioxidants are considered as important therapeutic tools for oxidative stress associated disorders as they can regulate the redox state, which is associated with cell and organ function. Inspired by natural polyphenols, six new caffeic acid sulfonamide derivatives were synthesized by coupling sulfonamides to the backbone of caffeic acid with good yields. Their structure and lipophilicity were characterized by 1H nuclear magnetic resonance (NMR), 13C{1H} NMR, infrared spectroscopy (IR) and oil-water partition coefficient assay. Their free radical scavenging activity and antioxidant activity were assessed by DPPH assay and hydrogen peroxide (H2O2) induced oxidative stress in human lung carcinoma A549 cells. The oil-water partition coefficient results indicate that the conjugation of sulfonamides increases the lipophilicity of caffeic acid. The CASMD, CASDZ and CASN results show higher free radical scavenging effects compared with vitamin C. The derivatives do not show any inhibitory effect on the proliferation of A549 cells up to a concentration of 200 μM, except CASDZ which significantly inhibits the growth of A549 cells at a concentration of 200 μM. In addition, the obtained derivatives markedly attenuate H2O2 induced decrease of cell viability, inhibit the production of ROS and MDA, and promote the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px). Besides, treatment of H2O2 stimulated A549 cells with caffeic acid sulfonamide derivatives further increases mRNA expression of NF-E2-related factor 2 (Nrf2) and its target genes, including heme oxygenase-1 (HO-1), NAD(P)H quinone dehydrogenase 1 (NQO1) and thioredoxin reductase 1 (TXNRD1). These results suggest that these new caffeic acid sulfonamide derivatives have higher lipophilicity and better antioxidant activities than the parent caffeic acid, and they might be able to control the antioxidant response in cells via the Nrf2 pathway.

p53 activation by Cr(VI): a transcriptionally limited response induced by ATR kinase in S-phase

Cellular reduction of carcinogenic chromium(VI) causes several forms of Cr-DNA damage with different genotoxic properties. Chromate-treated cultured cells have shown a strong proapoptotic activity of the DNA damage-sensitive transcription factor p53. However, induction of p53 transcriptional targets by Cr(VI) in rodent lungs was weak or undetectable. We examined Cr(VI) effects on the p53 pathway in human cells with restored levels of ascorbate that acts as a principal reducer of Cr(VI) in vivo but is nearly absent in standard cell cultures. Ascorbate-restored H460 and primary human cells treated with Cr(VI) contained higher levels of p53 and its Ser15 phosphorylation, which were induced by ATR kinase. Cr(VI)-stimulated p53 phosphorylation occurred in S-phase by a diffusible pool of ATR that was separate from the chromatin-bound pool targeting DNA repair substrates at the sites of toxic mismatch repair of Cr-DNA adducts. Even when more abundantly present than after exposure to the radiomimetic bleomycin, Cr(VI)-stabilized p53 showed a much more limited activation of its target genes in two types of primary human cells. No increases in mRNA were found for nucleotide excision repair factors and a majority of proapoptotic genes. A weak transcription activity of Cr(VI)-upregulated p53 was associated with its low lysine acetylation in the regulatory C-terminal domain, resulting from the inability of Cr(VI) to activate ATM in ascorbate-restored cells. Thus, p53 activation by ascorbate-metabolized Cr(VI) represents a limited genome-protective response that is defective in upregulation of DNA repair genes and proapoptotic transcripts for elimination of damaged cells.

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.

Chromium disrupts chromatin organization and CTCF access to its cognate sites in promoters of differentially expressed genes

Hexavalent chromium compounds are well-established respiratory carcinogens used in industrial processes. While inhalation exposure constitutes an occupational risk affecting mostly chromium workers, environmental exposure from drinking water is a widespread gastrointestinal cancer risk, affecting millions of people throughout the world. Cr(VI) is genotoxic, forming protein-Cr-DNA adducts and silencing tumor suppressor genes, but its mechanism of action at the molecular level is poorly understood. Our prior work using FAIRE showed that Cr(VI) disrupted the binding of transcription factors CTCF and AP-1 to their cognate chromatin sites. Here, we used two complementary approaches to test the hypothesis that chromium perturbs chromatin organization and dynamics. DANPOS2 analyses of MNase-seq data identified several chromatin alterations induced by Cr(VI) affecting nucleosome architecture, including occupancy changes at specific genome locations; position shifts of 10 nucleotides or more; and changes in position amplitude or fuzziness. ATAC-seq analysis revealed that Cr(VI) disrupted the accessibility of chromatin regions enriched for CTCF and AP-1 binding motifs, with a significant co-occurrence of binding sites for both factors in the same region. Cr(VI)-enriched CTCF sites were confirmed by ChIP-seq and found to correlate with evolutionarily conserved sites occupied by CTCF in vivo, as determined by comparison with ENCODE-validated CTCF datasets from mouse liver. In addition, more than 30% of the Cr(VI)-enriched CTCF sites were located in promoters of genes differentially expressed from chromium treatment. Our results support the conclusion that Cr(VI) exposure promotes broad changes in chromatin accessibility and suggest that the subsequent effects on transcription regulation may result from disruption of CTCF binding and nucleosome spacing, implicating transcription regulatory mechanisms as primary Cr(VI) targets.

Different roles of ROS and Nrf2 in Cr(VI)-induced inflammatory responses in normal and Cr(VI)-transformed cells

Hexavalent chromium (Cr(VI)) is classified as a human carcinogen. Cr(VI) has been associated with adenocarcinomas and squamous cell carcinoma of the lung. The present study shows that acute Cr(VI) treatment in human bronchial epithelial cells (BEAS-2B) increased inflammatory responses (TNF-α, COX-2, and NF-кB/p65) and expression of Nrf2. Cr(VI)-induced generation of reactive oxygen species (ROS) are responsible for increased inflammation. Despite the fact that Nrf2 is a master regulator of response to oxidative stress, silencing of Nrf2 in the acute Cr(VI) treatment had no effect on Cr(VI)-induced inflammation. In contrast, in Cr(VI)-transformed (CrT) cells, Nrf2 is constitutively activated. Knock-down of this protein resulted in decreased inflammation, while silencing of SOD2 and CAT had no effect in the expression of these inflammatory proteins. Results obtained from the knock-down of Nrf2 in CrT cells are very different from the results obtained in the acute Cr(VI) treatment. In BEAS-2B cells, knock-down of Nrf2 had no effect in the inflammation levels, while in CrT cells a decrease in the expression of inflammation markers was observed. These results indicate that before transformation, ROS plays a critical role while Nrf2 not in Cr(VI)-induced inflammation, whereas after transformation (CrT cells), Nrf2 is constitutively activated and this protein maintains inflammation while ROS not. Constitutively high levels of Nrf2 in CrT binds to the promoter regions of COX-2 and TNF-α, leading to increased inflammation. Collectively, our results demonstrate that before cell transformation ROS are important in Cr(VI)-induced inflammation and after transformation a constitutively high level of Nrf2 is important.

Mutagenic and carcinogenic actions of chromium and its compounds

Numerous experimental observations have been made on microorganisms and culture of the cells of mammals as well as the accounting of the chromosomal aberrations in the bone marrow cells of the mammals and of human cells displayed that the chromium and its compounds possess a pronounced mutagenic effect. Translocation test, induction record of DNA damage and repair systems in the mammalian and human cells with greater precision proves the presence of the mutagenic effect of the chromium and its compounds, which in turn is dependent on dose and time of this metal intoxication. Chromium and its compounds have pronounced mutagenic effect, on increased admission to organism of mammals and protozoa.

DNA double-strand breaks by Cr(VI) are targeted to euchromatin and cause ATR-dependent phosphorylation of histone H2AX and its ubiquitination

Hexavalent chromium is a human respiratory carcinogen that undergoes intracellular activation in vivo primarily via reduction with ascorbate. Replication of Cr-adducted DNA triggers mismatch repair that generates toxic DNA double-strand breaks (DSBs) as secondary lesions. Here, we examined the intranuclear distribution of chromate-induced breaks and a central DSB signaling branch targeting histone H2AX. Using ascorbate-restored cells (H460 human lung epithelial cells, normal human lung and normal mouse embryonic fibroblasts (MEFs)), we found that Cr(VI) produced a typical DSB-associated spectrum of H2AX modifications, including its Ser139-phosphorylated (known as γH2AX) and mono- and diubiquitinated forms. However, whereas canonical DSB signaling relies on ATM, the formation of γH2AX and its ubiquitinated products by Cr(VI) was dependent on ATR kinase. Based on the established mode of ATR activation, this suggests that Cr-induced DSB are not blunt-ended and likely contain single-stranded tails. Confocal imaging with markers of active and inactive chromatin revealed a selective formation of Cr-induced DSB in euchromatin of mouse and human cells. In contrast to DSB, Cr-DNA adducts were produced in both types of chromatin. The euchromatin targeting of Cr-induced DSB makes these lesions particularly dangerous by increasing the probability of deleting active tumor suppressors and producing oncogenic translocations. Accumulation of transcription-inhibiting ubiquitinated forms of γH2AX in euchromatin is expected to contribute to the ability of Cr(VI) to suppress upregulation of inducible genes.

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.

Chromium (VI) inhibits mouse metallothionein-I gene transcription by modifying the transcription potential of the co-activator p300

The production of the heavy metal-binding proteins, the metallothioneins (MTs), is induced by heavy metals such as Zn, Cd, and Hg. MTs maintain Zn homeostasis and attenuate heavy metal-induced cytotoxicity by sequestering these metals and lowering their intracellular concentrations. Previously, we had reported that Zn induced the formation of a co-activator complex containing metal response element-binding transcription factor-1 (MTF-1) and the histone acetyltransferase (HAT), p300, which plays an essential role in the activation of MT-1 transcription. In addition, we had shown that Cr(VI) inhibits Zn-induced MT-1 transcription by preventing the Zn-dependent formation of the MTF-1-p300 complex. In the current study, we have shown that the inhibition by Cr(VI) was partially overcome by the overexpression of p300 or MTF-1 in an MT-I promoter-driven luciferase reporter assay system and have used real-time RT-PCR to determine MT-I mRNA levels. It has been reported that Cr(VI) inhibits CYP1A1 transcription by crosslinking histone deacetylase (HDAC) to the promoter. The crosslink inhibits the recruitment of p300 to the MT-1 promoter and blocks HAT-dependent transactivation by p300. However, our results demonstrate that trichostatin A, an HDAC inhibitor, could not block the inhibitory effects of Cr(VI) on MT-1 transcription and that there were no significant differences in the in vitro inhibitory effects of Cr(VI), Cr(III), and Zn on p300 HAT activity. This suggests that the inhibitory effects of Cr(VI) on MT-I transcription may be due to its effects on the HAT-independent transactivation ability rather than the HAT-dependent, HDAC release-related transactivation ability of p300.

Chromium genotoxicity: A double-edged sword

Certain forms of hexavalent chromium [Cr(VI)] are known respiratory carcinogens that induce a broad spectrum of DNA damage. Cr(VI)-carcinogenesis may be initiated or promoted through several mechanistic processes including, the intracellular metabolic reduction of Cr(VI) producing chromium species capable of interacting with DNA to yield genotoxic and mutagenic effects, Cr(VI)-induced inflammatory/immunological responses, and alteration of survival signaling pathways. Cr(VI) enters the cell through non-specific anion channels, and is metabolically reduced by agents including ascorbate, glutathione, and cysteine to Cr(V), Cr(IV), and Cr(III). Cr(III) has a weak membrane permeability capacity and is unable to cross the cell membrane, thereby trapping it within the cell where it can bind to DNA and produce genetic damage leading to genomic instability. Structural genetic lesions produced by the intracellular reduction of Cr(VI) include DNA adducts, DNA-strand breaks, DNA-protein crosslinks, oxidized bases, abasic sites, and DNA inter- and intrastrand crosslinks. The damage induced by Cr(VI) can lead to dysfunctional DNA replication and transcription, aberrant cell cycle checkpoints, dysregulated DNA repair mechanisms, microsatelite instability, inflammatory responses, and the disruption of key regulatory gene networks responsible for the balance of cell survival and cell death, which may all play an important role in Cr(VI) carcinogenesis. Several lines of evidence have indicated that neoplastic progression is a result of consecutive genetic/epigenetic changes that provide cellular survival advantages, and ultimately lead to the conversion of normal human cells to malignant cancer cells. This review is based on studies that provide a glimpse into Cr(VI) carcinogenicity via mechanisms including Cr(VI)-induced death-resistance, the involvement of DNA repair mechanisms in survival after chromium exposure, and the activation of survival signaling cascades in response to Cr(VI) genotoxicity.

Chromium(VI) stimulates Fyn to initiate innate immune gene induction in human airway epithelial cells

Mechanisms for pathogenic metal signaling in airway injury or disease promotion are poorly understood. It is widely believed that one mechanism for pathogenic and possible carcinogenic effects of inhaled chromium (Cr(VI)) is inhibition of inducible gene transactivation. However, we recently reported that Cr(VI) inhibition of Sp1-dependent transactivation required signal transducer and activator of transcription 1 (STAT1)-dependent expression of an inhibitory protein in airway epithelium. Thus, Cr(VI) exposures can induce genes, and we hypothesized that this induction resulted from Cr(VI) signaling through an innate immune-like STAT1-dependent pathway initiated by Fyn. Exposure of human airway epithelial (BEAS-2B) cells to Cr(VI) selectively transactivated the STAT-responsive interferon-stimulated response element (ISRE) and induced ISRE-driven transactivation of interferon regulatory factor 7 (IRF7), without affecting the gamma interferon-activated site (GAS)-driven IRF1 expression. Cr(VI)-induced IRF7 was absent or greatly reduced in cells that lacked STAT1, were treated with the Src family kinase inhibitor, PP2, or lacked Fyn. Expressing Fyn, but not Src, in mouse embryonic fibroblasts cells null for Src, Yes, and Fyn restored Cr(VI)-stimulated STAT1 tyrosine phosphorylation and IRF7 expression. Finally, shRNA knockdown of Fyn in BEAS-2B cells prevented Cr(VI)-activated STAT1 transactivation of IRF7. These data support a novel mechanism through which Cr(VI) stimulates Fyn to initiate interferon-like signaling for STAT1-dependent gene transactivation.

Signal transducer and activator of transcription 1 (STAT1) is essential for chromium silencing of gene induction in human airway epithelial cells

Hexavalent chromium (Cr(VI)) promotes lung injury and pulmonary diseases through poorly defined mechanisms that may involve the silencing of inducible protective genes. The current study investigated the hypothesis that Cr(VI) actively signals through a signal transducer and activator of transcription 1 (STAT1)-dependent pathway to silence nickel (Ni)-induced expression of vascular endothelial cell growth factor A (VEGFA), an important mediator of lung injury and repair. In human bronchial airway epithelial (BEAS-2B) cells, Ni-induced VEGFA transcription by stimulating an extracellular regulated kinase (ERK) signaling cascade that involved Src kinase-activated Sp1 transactivation, as well as increased hypoxia-inducible factor-1 alpha (HIF-1 alpha) stabilization and DNA binding. Ni-stimulated ERK, Src, and HIF-1 alpha activities, as well as Ni-induced VEGFA transcript levels were inhibited in Cr(VI)-exposed cells. We previously demonstrated that Cr(VI) stimulates STAT1 to suppress VEGFA expression. In BEAS-2B cells stably expressing STAT1 short hairpin RNA, Cr(VI) increased VEGFA transcript levels and Sp1 transactivation. Moreover, in the absence of STAT1, Cr(VI), and Ni coexposures positively interacted to further increase VEGFA transcripts. This study demonstrates that metal-stimulated signaling cascades interact to regulate transcription and induction of adaptive or repair responses in airway cells. In addition, the data implicate STAT1 as a rate limiting mediator of Cr(VI)-stimulated gene regulation and suggest that cells lacking STAT1, such as many tumor cell lines, have opposite responses to Cr(VI) relative to normal cells.

Chromium(VI) inhibits mouse metallothionein-I gene transcription by preventing the zinc-dependent formation of an MTF-1-p300 complex

Mouse MT-I (metallothionein-I) transcription is regulated by MTF-1 (metal-response-element-binding transcription factor-1) which is recruited to the promoter in response to zinc. Cr(VI) [chromium(VI)] pretreatment blocks zinc-activation of the endogenous MT-I gene and attenuates zinc-activation of MT-I-promoter-driven luciferase reporter genes in transient transfection assays. Chromatin immunoprecipitation assays revealed that Cr(VI) only modestly reduces recruitment of MTF-1 to the MT-I promoter in response to zinc, but drastically reduces the recruitment of RNA polymerase II. These results suggest that Cr(VI) inhibits the ability of MTF-1 to transactivate this gene in response to zinc. Zinc has recently been shown to induce the formation of a co-activator complex containing MTF-1 and the histone acetyltransferase p300 which plays an essential role in the activation of MT-I transcription. In the present study, co-immunoprecipitation assays demonstrated that Cr(VI) pretreatment blocks the zinc-induced formation of this co-activator complex. Thus Cr(VI) inhibits mouse MT-I gene expression in response to zinc by interfering with the ability of MTF-1 to form a co-activator complex containing p300 and recruiting RNA polymerase II to the promoter.

[Molecular mechanism involved in chromium(VI) toxicity]

Chromium exists in many different oxidation states in the environment, Cr(VI) and Cr(III) being the most stable forms. Chromium has been known for over 100 years to be a human carcinogen. The greatest risk of cancer from chromium exposure is associated with Cr(VI). Cr(VI) enters cells via the sulfate anion transporter system and is reduced to intermediate oxidation states, such as Cr(V) and Cr(IV), in the process of forming stable Cr(III) forms. It is known that Cr(VI) affects expression of various genes. Metal responsive element-binding transcription factor-1 (MTF-1) is involved in sensing heavy metal load and the induced transcription of several protective genes, including metallothionein (MT)-I, MT-II, zinc transporter-1, and gamma-glutamylcysteine synthetase. Cr(VI) inhibits zinc-induced MT transcription via modifying transactivation potential of MTF-1. However, the molecular mechanism for the Cr(VI)-mediated inhibition of MTF-1 has not been fully elucidated. In this review, I briefly summarize the previous studies and discuss the current status of research on Cr(VI) toxicity and Cr(VI)-mediated inhibition against transcription.

Cr(VI)-stimulated STAT3 tyrosine phosphorylation and nuclear translocation in human airway epithelial cells requires Lck

Chronic inhalation of low amounts of Cr(VI) promotes pulmonary diseases and cancers through poorly defined mechanisms. SFKs (Src family kinases) in pulmonary airway cells may mediate Cr(VI) signalling for lung injury, although the downstream effectors of Cr(VI)-stimulated SFKs and how they relate to pathogenic gene induction are unknown. Therefore SFK-dependent activation of transcription factors by non-cytotoxic exposure of human bronchial epithelial cells to Cr(VI) was determined. Protein-DNA binding arrays demonstrated that exposing BEAS 2B cells to 5 microM Cr(VI) for 4 and 24 h resulted in increased protein binding to 25 and 43 cis-elements respectively, while binding to 12 and 16 cis-elements decreased. Of note, Cr(VI) increased protein binding to several STAT (signal transducer and activator of transcription) cis-elements. Cr(VI) stimulated acute tyrosine phosphorylation and nuclear translocation of STAT1 over a 4 h period and a prolonged activation of STAT3 that reached a peak between 48 and 72 h. This prolonged activation was observed for both STAT3alpha and STAT3beta. Immunofluorescent confocal microscopy confirmed that Cr(VI) increased nuclear localization of phosphorylated STAT3 for more than 72 h in both primary and BEAS 2B human airway cells. Cr(VI) induced transactivation of both a STAT3-driven luciferase reporter construct and the endogenous inflammatory gene IL-6 (interleukin-6). Inhibition with siRNA (small interfering RNA) targeting the SFK Lck, but not dominant-negative JAK (Janus kinase), prevented Cr(VI)-stimulated phosphorylation of both STAT3 isoforms and induction of IL-6. The results suggest that Cr(VI) activates epithelial cell Lck to signal for prolonged STAT3 activation and transactivation of IL-6, an important immunomodulator of lung disease progression.

Genetic and cellular mechanisms in chromium and nickel carcinogenesis considering epidemiologic findings

Genetic and environmental interactions determine cancer risks but some cancer incidence is primarily a result of inherited genetic deficits alone. Most cancers have an occupational, viral, nutritional, behavioral or iatrogenic etiology. Cancer can sometimes be controlled through broad public health interventions including industrial hygiene and engineering controls. Chromium and nickel are two human carcinogens associated with industrial exposures where public health measures apparently work. Carcinogenic mechanisms of these metals are examined by electron-spin-resonance-spectroscopy and somatic-mutation-and-recombination in Drosophila melanogaster in this report. Both metals primarily affect initiation processes in cancer development suggesting important theoretical approaches to prevention and followup.

Mechanisms of chromium-induced suppression of RNA synthesis in cellular and cell-free systems: relationship to RNA polymerase arrest

Chromium(VI) (Cr(VI)) can suppress both DNA replication and transcription as a result of chromium (Cr)-induced DNA damage. While progress has been made in the characterization of Cr-induced DNA polymerase arresting lesions, very little information is available on the inhibition of transcription by this metal. The aim of the present study was to identify the molecular mechanisms involved in the reduction of RNA synthesis by Cr. Following treatment with a moderately cytotoxic dose (approximately LC50) of Cr(VI) (150 microM for 2 h), total RNA synthesis was initially suppressed in CHO cells and recovered to control levels within 72 h post-treatment. In vitro nuclear run-on transcription assays of nuclei isolated from Cr(VI)-treated cells showed a similar amount of RNA synthesis suppression as observed in intact cells. Qualitative analysis of nascent transcripts revealed a general, concentration-dependent reduction in size suggesting that transcriptional elongation was inhibited following Cr-treatment. Transcriptional initiation in these nuclei was also reduced. To better determine whether transcriptional suppression was related to Cr-induced DNA damage we examined the transcriptional activity of T7 RNA polymerase on Cr(III)-treated plasmid DNA. Treatment of pGEM3Z-TS DNA with Cr(III) resulted in transcriptional arrest which occurred primarily at GC-rich and palindromic regions. However, in contrast to the cellular data, transcriptional initiation was unaffected in the in vitro transcription arrest assays. Taken together, these results suggest that the suppression of RNA synthesis by Cr is related to chromium-induced template DNA damage which prevents elongation leading to premature RNA polymerase arrest.

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.

Characterization of nonmutagenic Cr(III)-DNA interactions

Exposure of cells or animals to carcinogenic chromium(VI) (Cr(VI)) produces Cr(III)-DNA adducts. The relevance of these lesions to Cr(VI)-induced tumors is unclear. Various Cr(III) complexes have been used to model the products resulting from Cr(VI) metabolism in order to gain mechanistic insights. The purpose of this study was to characterize interactions of Cr(III) complexes with DNA in order to evaluate their use as models for these purposes. The reactivity of DNA with chromic chloride hexahydrate (CrCl(3)) and sodium bis(l-cysteinato)chromium(III) dihydrate (Cr(cys)(2)(-)) was compared to that with cis-diamminedichloroplatinum(II) (cis-platin). Both Cr(III) and Pt(II) cause unwinding of supercoiled DNA that can be visualized as a mobility shift by gel electrophoresis. Chromic chloride was much less distorting than cis-platin, unwinding DNA by only 1-2 degrees, and Cr(cys)(2)(-) interacted with DNA only weakly. Consistent with in vitro studies, CrCl(3) produced Cr-DNA adducts in CHO AA8 cells at levels equivalent to those obtained with Cr(VI), whereas Cr(cys)(2)(-) did not produce significant adducts. Lesions produced by CrCl(3) were not mutagenic in the hypoxanthine-Gua-phosphoribosyl-transferase assay. These data are consistent with CrCl(3) producing a nondistorting lesion, perhaps by association with the phosphate backbone. There are two possible interpretations of these results: Either the Cr(III) products formed by Cr(VI) metabolism are not modeled by CrCl(3) and Cr(cys)(2)(-) complexes, or Cr(III) is not an active species for Cr(VI)-induced DNA damage. This study provides the first structural evidence for Cr(III)-DNA adducts. A molecular understanding of Cr(III)-DNA interactions will be necessary before we can determine their relevance in Cr(VI)-induced cancers.

Combining Drosophila melanogaster somatic-mutation-recombination and electron-spin-resonance-spectroscopy data to interpret epidemiologic observations on chromium carcinogenicity

Lung cancers are significantly increased among workers exposed to chromate (Cr6+, Cr3+), chromium pigments (Cr6+) and chromium plating (Cr6+). Chromium lung burdens and cancer risk increase proportionately with duration of employment at long latencies. However, this epidemiologic information alone is insufficient in determining whether Cr6+ or Cr3+ are equally important in causing cancer. We have attempted to combine epidemiologic data with data from the Drosophila melanogaster somatic-mutation-recombination-test and from the in vitro electron-spin-resonance spectroscopy study to demonstrate that following somatic recombination plays a more important role than somatic mutation in chromium carcinogenesis. Cr4+ is more important than Cr5+ or Cr6+ in inducing somatic recombination while Cr6+ produces more and bigger clones than Cr4+ in somatic mutation. Cr3+ produces negative results in this fruit-fly wing-spot-assay. When the larvae and flies exposed to Cr6+ and Cr4+ are examined by ESR, only Cr5+ and Cr3+ are found. Thermodynamic parameters deltaE, deltaH, and deltaS are also estimated from these latter experiments to explain the relative importance of Cr6+, Cr4+, Cr3+ in chromium carcinogenesis among exposed industrial workers.

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.

Chromium(VI) inhibits the transcriptional activity of nuclear factor-kappaB by decreasing the interaction of p65 with cAMP-responsive element-binding protein-binding protein

Chromium(VI) regulation of gene expression has been attributed to the generation of reactive chromium and oxygen species, DNA damage, and alterations in mRNA stability. However, the effects of Cr(VI) on signal transduction leading to gene expression are not resolved. Therefore, this study investigated the effects of Cr(VI) on basal and tumor necrosis factor-alpha (TNF-alpha)-induced transcriptional competence of nuclear factor-kappaB (NF-kappaB) in A549 human lung carcinoma cells. Pretreatment of A549 cells with nontoxic levels of Cr(VI) inhibited TNF-alpha-stimulated expression of the endogenous gene for interleukin-8 and of an NF-kappaB-driven luciferase gene construct, but not expression of urokinase, a gene with a more complex promoter. Chromium did not inhibit TNF-alpha-stimulated IkappaBalpha degradation or translocation of NF-kappaB-binding proteins to the nucleus. However, Cr(VI) pretreatments prevented TNF-alpha-stimulated interactions between the p65 subunit of NF-kappaB and the transcriptional cofactor cAMP-responsive element-binding protein-binding protein (CBP). This inhibition was not the result of an effect of chromium on the protein kinase A catalytic activity required for p65/CBP interactions. In contrast, Cr(VI) caused concentration-dependent increases in c-Jun/CBP interactions. These data indicate that nontoxic levels of hexavalent chromium selectively inhibit NF-kappaB transcriptional competence by inhibiting interactions with coactivators of transcription rather than DNA binding.

Inhibition of protein synthesis by chromium(VI) differentially affects expression of urokinase and its receptor in human type II pneumocytes

Exposure to chromium(VI) increases the incidence of cancer, respiratory distress, and pulmonary fibrosis. The latter is a pathological disorder characterized by decreased urokinase-type plasminogen activator (uPA) activity and fibrinolysis. In this study, treatment of alveolar type II cells (A549) with 1 to 5 microM chromium(VI) for 4 and 12 h decreased both the specific activity and the amount of uPA protein. Chromium reduced uPA protein levels by inhibiting protein synthesis and had no effect on uPA mRNA levels or the rate of uPA protein degradation. In contrast, both mRNA and protein levels for the uPA receptor (uPAR) were increased by treatment with concentrations of chromium(VI) that did not completely inhibit protein synthesis. The chromium-induced increase in uPAR resulted from increased message stability. These data indicate that chromium has differential effects on expression of the proteins in the pulmonary fibrinolytic cascade. The net loss of uPA activity may promote fibrosis following inhalation of chromium(VI).

Heavy metals mercury, cadmium, and chromium inhibit the activity of the mammalian liver and kidney sulfate transporter sat-1

Heavy metal intoxication leads to defects in cellular uptake mechanisms in the mammalian liver and kidney. We have studied the effects of several heavy metals, including mercury, lead, cadmium, and chromium (at concentrations of 1 to 1000 microM), on the activity of the mammalian sulfate transporter sat-1(2) in Xenopus oocytes. sat-1 encodes a sulfate/bicarbonate anion exchanger expressed in the rat liver and kidney. Mercury (10 microM) strongly inhibited sat-1 transport by reducing Vmax by eightfold but not its Km for inorganic sulfate (Si). Lead (up to 1 mM) was unable to significantly inhibit sat-1 transporter activity. Cadmium (500 microM) showed weak inhibition of sat-1 transport by decreasing only sat-1 Vmax. Chromium (100 microM) strongly inhibited sat-1 transport by reducing Km for Si by sevenfold, most probably by binding to the Si site, due to the strong structural similarity between the CrO2-4 and SO2-4 substrates. This study presents the first characterization of heavy metal inhibition of the hepatic and renal sulfate/bicarbonate transporter sat-1, through various mechanisms, which may lead to sulfaturia following heavy metal intoxication.

Mechanism of the synergistic induction of CYP2H by isopentanol plus ethanol: comparison to glutethimide and relation to induction of 5-aminolevulinate synthase

We had previously found that combined treatment with isopentanol and ethanol synergistically induced CYP2H protein and activity in cultured chick nepatoytes. Here we investigated the mechanism of induction of CYP2H by the alcohols and whether they caused a coordinate induction of 5-aminolevulinate synthase (ALAS) mRNA. Treatment with isopentanol alone or in combination with ethanol resulted in coordinate increases in CYP2H1 and ALAS mRNAs. With isopentanol alone, the amounts of CYP2H1 and ALAS mRNAs at 4 to 6 h were similar to those observed after treatment with the alcohol combination, but declined by 11 h. Readdition of isopentanol at 11 h again increased the expression of both mRNAs, indicating that the decreases at 11 h were due to limiting amounts of inducer. Similar results were observed in cells exposed to low concentrations of glutethimide. In the combined alcohol treatment, increases in CYP2H1 and ALAS mRNAs were sustained from 4 h to 11 h after addition of the alcohols, but decreased to control levels by 24 h. Using pulse labeling to measure de novo synthesis of CYP2H1/2 protein, we found that the increases in CYP2H1/2 protein reflected the increases in CYP2H1 mRNA. The half-life of CYP2H1/2 protein, measured from pulse-chase experiments, was approximately twofold greater than the half-life of CYP2H1 mRNA. Our results indicate that the alcohols and glutethimide coordinately increase ALAS and CYP2H1 mRNA, and that increases in CYP2H1/2 protein arise from increases in its mRNA.

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.

Developmental regulation of the 3-methylcholanthrene- and dioxin-inducible CYP1A5 gene in chick embryo liver in vivo

The cDNA sequences for two dioxin-inducible cytochrome P450s in chicken, CYP1A4 and CYP1A5, have recently been reported which correspond to two dioxin-inducible forms of P450 previously designated as TCDDAHH and TCDDAA, respectively. The developmental expression of CYP1A4-associated aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) activity and its association with expression of the Ah receptor had previously been characterized in chick embryo liver. The purpose of this study was to examine the developmental regulation of the second dioxin-inducible P450 gene, CYP1A5, in chick embryo liver. A partial gene sequence for CYP1A5 indicated that the intron/exon organization of this gene was identical to that of the CYP1A1 and CYP1A2 mammalian genes and was present in a single copy in the genome. CYP1A5 mRNA was expressed basally in chick embryo liver and was highly inducible by the Ah receptor ligands, 3-methylcholanthrene, beta-naphthoflavone, and 3,4,3', 4'-tetrachlorobiphenyl (TCB), but not by the phenobarbital analog, glutethimide. CYP1A5 mRNA levels were increased 40- to 50-fold within 5 h after a single TCB treatment, corresponding to a 30- to 40-fold increase in the transcription rate of the CYP1A5 gene at this time point. In contrast to a previous report that CYP1A5 mRNA expression was inducible by estradiol, we observed no effects of estradiol or dexamethasone on CYP1A5 mRNA expression, either alone or in combination with TCB. Basal and TCB-inducible CYP1A5 mRNA expression was maximal in liver at 8 days of development and remained high throughout the remainder of embryonic development. Thus, CYP1A5 appears to be regulated in a very similar manner to CYP1A4 in chick embryo liver.

Induction of apoptotic cell death by particulate lead chromate: differential effects of vitamins C and E on genotoxicity and survival

Certain hexavalent chromium compounds are documented human carcinogens. Exposure of cells to particulate forms of chromium results in cell-enhanced dissolution of particles in the extracellular microenvironment and chronic production of chromium oxyanions, which are taken up by the cell through an anion transport system and are genotoxic and clastogenic. It was previously shown that apoptosis is the mode of cell death of nearly all of the Chinese hamster ovary cells (CHO-AA8 cell line), which die after high-dose, short-term treatments with soluble sodium chromate. In this report the mode of cell killing by particulate lead chromate and of low-dose continuous treaments of soluble sodium chromate designed to mimic conditions of ionic chromate uptake after lead chromate exposure was examined. CHO-AA8 cells were treated for 24 hr with doses of sodium chromate or lead chromate which cause a 50% decrease in survival in colony-forming effeciency assays. Longer treatments (up to 72 hr) at the same doses did not decrease survival further than the 24-hr exposure. Morphological changes indicative of apoptosis, as well as internucleosomal DNA fragmentation, were detectable by 24 hr after treatment with lead chromate or soluble sodium chromate. All of the cells killed by treatments with lead chromate particles underwent apoptosis as the mode of cell death and this was accurately modeled in cell culture by continuous treatments with low-dose soluble sodium chromate. Exposure of cells to hexavalent chromium compounds causes a spectrum of DNA damage which can be selectively altered by pretreatment of cells with antioxidant vitamins prior to chromium exposure. Here we show that ascorbate and alpha-tocopherol markedly inhibited the chromosomal aberrations induced by both particulate and soluble chromate compounds, even though chromium adduct levels were not decreased by either vitamin pretreatment. Cell survival assays showed that ascorbate, but not alpha-tocopherol, protected cells from apoptosis induced by sodium chromate. The results differentiate chromium-induced apoptosis from both chromosomal damage and adduct levels and suggest that other lesions sensitive to ascorbate but not tocopherol are the proximal inducing signal for chromium-induced apoptosis.

Mutational spectrum induced by chromium(III) in shuttle vectors replicated in human cells: relationship to Cr(III)-DNA interactions

Trivalent chromium (Cr(III)), the ultimate species of chromium (VI) intracellular reduction, can associate with DNA forming Cr(III) monoadducts and DNA-DNA cross-links. However, the mutational specificity of Cr(III) has not been determined partly because Cr(III) has difficulty entering cells. In this study, we have characterized the types of Cr(III)-induced DNA lesions in two buffer systems and the mutational spectrum of Cr(III)-treated shuttle vectors replicated in human 293 cells. Plasmids were treated with Cr(III) in buffers consisting of either 10 mM potassium phosphate, pH 7.5 (designated as KP), or 0.2 mM Tris-HCl and 20 microM EDTA, pH 7.4 (designated as TE/50). The amounts of Cr(III) bound to DNA increased as Cr(III) concentration increased in both buffers; these Cr(III)-DNA associations were stable in both buffers during a 24-h dialysis. The electrophoretic mobility of supercoiled DNA was markedly retarded in samples treated with Cr(III) in TE/50 but not KP buffer, suggesting that Cr(III)-mediated DNA-DNA cross-links were generated in TE/50 but did not form in KP. Polymerase-stop assay showed that DNA polymerases were mostly blocked at the 3' adjacent bases of guanines on templates treated with Cr(III) in TE/50 but were not observed on those treated in KP. The signals of Cr(III)-mediated cross-links generated in TE/50 buffers were reduced when they were dialyzed against KP buffers. Similarly, Cr(III)-DNA monoadducts formed in KP were converted to primer-template cross-links by dialysis against TE/50. The mutation frequency of Cr(III) in the supF gene of pSP189 or pZ189 shuttle vectors replicated in human cells increased as Cr(III) concentration increased in both buffers. DNA sequencing analysis showed that single-base substitutions (61-68%), two-base substitutions (3-5%), and deletions (21-34%) were induced in similar frequencies in plasmids treated with Cr(III) in either TE/ 50 or KP. The Cr(III)-induced base-substitution hot spots are different from those occurring spontaneously. Cr(III) enhances G.C base substitutions, particularly G.C-->C.G transversions, at 5'GA, 5'CG, and 5'AG sites. Base-substitution hot spots did not correlate with strong polymerase-stop sites, suggesting that base substitutions are derived from Cr(III) monoadducts, not from DNA-DNA cross-links.

Mechanisms of the carcinogenic chromium(VI)-induced DNA-protein cross-linking and their characterization in cultured intact human cells

DNA-protein complexes (DPCs) were induced in human leukemic T-lymphocyte MOLT4 cells by treatment with potassium chromate. DPCs were isolated by ultracentrifugal sedimentation in the presence of 2% SDS and 5 M urea. The complexes were analyzed by two-dimensional SDS-polyacrylamide gel electrophoresis. Three acidic proteins of 74, 44, and 42 kDa and a basic protein of 51 kDa were primarily complexed to DNA following 25 microM chromate treatment. Higher concentrations of chromate cross-linked many other proteins to DNA. Amino acid sequencing and immunoblotting studies indicated that the acidic 44-kDa protein could be nuclear beta-actin. Lectin and aminoglycoside nucleotidyltransferase were also found to cross-link with DNA by chromate treatment. The composition and stability of the DPCs were studied using nucleases, proteinase K, and disruptive chemicals. Pretreatment of cells with antioxidants inhibited the formation of DPCs, measured as K+-SDS precipitable DPCs, indicating the involvement of oxidative mechanisms. Because chromate causes certain nuclear proteins to form complexes with DNA and the complexes are resistant to treatments such as 2% SDS and 5 M urea, but disruptable under gel electrophoretic conditions, chromium could be used as a cross-linking agent for the identification of other proteins, such as transcription factors, that transiently interact with DNA.

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.

Long-term exposure to chromium(VI) oxide leads to defects in sulfate transport system in Chinese hamster ovary cells

Chromium(VI) resistant Chinese hamster ovary (CHO) cell lines were established in this study by exposing parental CHO-K1 cells to sequential increases in CrO3 concentration. The final concentration of CrO3 used for selection was 7 microM for Cr7 and 16 microM for Cr16 cells. Cr16-1 was a subclone derived from Cr16 cells. Next, these resistant cells were cultured in media without CrO3 for more than 6 months. The resistance of these cells to CrO3 was determined by colony-forming ability following a 24-h treatment. The LD50 of CrO3 for chromium(VI) resistant cells was at least 25-fold higher than that of the parental cells. The cellular growth rate, chromosome number, and the hprt mutation frequency of these chromium(VI) resistant cells were quite similar to their parental cells. The glutathione level, glutathione S-transferase, catalase activity, and metallothionine mRNA level in Cr7 and Cr16-1 cells were not significantly different from their parental cells. Furthermore, Cr16-1 cells were as sensitive as CHO-K1 cells to free-radical generating agents, including hydrogen peroxide, nickel chloride, and methanesulfonate methyl ester, and emetine, i.e., a protein synthesis inhibitor. The uptake of chromium(VI) and the remaining amount of this metal in these resistant and the parental cell lines were assayed by atomic absorption spectrophotometry. Experimental results indicated that a vastly smaller amount of CrO3 entered the resistant cell lines than their parental cells did. A comparison was made of the sulfate uptake abilities of CHO-K1 and chromium(VI) resistant cell lines. These results revealed that the uptake of sulfate anion was substantially reduced in Cr7 and Cr16-1 cells. Extracellular chloride reduced sulfate uptake in CHO-K1 but not in Cr16-1 cells. Therefore, the major causative for chromium(VI) resistance in these resistant cells could possibly be due to the defects in SO4(2-)/C1- transport system for uptake chromium(VI).

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.

Possible role of oxidative damage in metal-induced carcinogenesis

This review presents and evaluates evidence relevant to the mechanisms of metal carcinogenicity with special emphasis on the emerging hypothesis of the oxidative nature of metals' effect on DNA. The carcinogenic transition metals are capable of in vivo binding with the cell nucleus and causing promutagenic damage that includes DNA base modifications, inter- and intramolecular crosslinking of DNA and proteins, DNA strand breaks, rearrangements, and depurination. The chemistry of that damage and the resulting mutations observed in vitro and in metal-induced tumors are both characteristic for oxidative attack on DNA. The underlying mechanism involves various kinds of active oxygen and other radical species arising from metal-catalyzed redox reactions of O2, H2O2, lipid peroxides, and others, with certain amino acids, peptides, and proteins. Other metal-mediated pathogenic effects, such as enhancement of lipid peroxidation, stimulation of inflammation, inhibition of cellular antioxidant defenses, and inhibition of DNA repair, may also contribute to that mechanism. Thus far, published data revealing the oxidative character of metal-induced promutagenic DNA alterations are particularly strong for two of the most powerful human metal carcinogens, chromium and nickel. However, without excluding contribution of other effects, the promotion of oxidative damage tends to take the leading role in explaining mechanisms of carcinogenicity and acute toxicity of certain other metals as well.

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

Previous studies have shown that a number of different genotoxic carcinogens that induce different types of DNA damage preferentially alter the expression of inducible genes in vivo. To investigate further the mechanistic basis for these effects, we examined the effects of the human lung carcinogen chromium(VI) on expression of the hormone-inducible cytosolic phosphoenolpyruvate carboxykinase (PEPCK) gene in chick embryo liver. Chromium(VI) pretreatment had significant effects on both basal and glucocorticoid-inducible PEPCK expression in 14-d-old embryo liver. These effects were principally a result of changes in PEPCK transcription. In contrast, treatment with chromium(VI) 1 h after treatment with glucocorticoid had no effect on PEPCK induction, suggesting that an early event in the induction process is the target for carcinogen effects. In 16-d-old liver, in which PEPCK expression is no longer responsive to glucocorticoid induction, both basal and inducible PEPCK expression were also refractory to chromium(VI) effects, indicating that carcinogen responsiveness is a phenotypic rather than an inherent property of inducible genes and is related to their competence for induction. Chromium(VI) had no effect on cAMP induction of PEPCK expression, demonstrating that carcinogens target their effects to specific regulatory pathways. Comparison of the effects of chromium(VI) with those of cycloheximide suggests that chromium(VI) targets its effects to a labile, constitutively expressed repressor involved in PEPCK gene regulation.

Comparison of effects of direct-acting DNA methylating and ethylating agents on inducible gene expression in vivo

Our laboratory is interested in whether chemical carcinogen-induced DNA damage is non-randomly distributed in the genome, i.e., "targeted," at the level of individual genes. As one means of investigating this, we have examined whether carcinogen treatment differentially alters the expression of specific genes in vivo. In this study, we have compared the effects of four direct-acting simple alkylating agents (methyl methanesulfonate, ethyl methanesulfonate, methylnitrosourea, and ethylnitrosourea) on the steady-state mRNA expression of a model inducible gene, phosphoenolpyruvate carboxykinase (PEPCK), using the chick embryo as a simple in vivo test system. We observed no effect of any of these four carcinogens on the steady-state mRNA expression of the constitutively expressed beta-actin, transferrin, or albumin genes in chick embryo liver following a single dose of carcinogen. In contrast, these same treatments significantly altered both the basal and inducible expression of the glucocorticoid-inducible PEPCK gene. These results support the hypothesis that inducible gene expression is a target for the effects of chemical carcinogens in vivo. In addition, the direction, magnitude, and time course of these effects were agent-specific. Qualitative and quantitative differences in effects between the methylating and ethylating agents and between the methanesulfonates and nitrosoureas were correlated with differences in their specific patterns of DNA adduct formation, suggesting that different DNA lesions have different effects on inducible gene expression.

In vivo inhibition of glucocorticoid-inducible gene expression by dimethylnitrosamine in rat liver

Sprague-Dawley rats were pretreated with a single i.p. injection of either 2.25 mL/kg of phosphate-buffered saline (PBS) or 22.5 mg/kg of dimethylnitrosamine (DMN) followed 2 hr later by a single i.p. injection of either 1.35 mg/kg of dexamethasone (DEX) or the vehicle, a 50% ethanol solution, both delivered in a volume of 3 mL/kg. RNA levels of the hormone-inducible, specialized liver function genes, tyrosine aminotransferase (TAT) and glutamine synthetase (GS), were monitored 4, 5, 6, 7, 8, and 10 hr after the DEX injection. Maximal induction of both the TAT (26-fold) and GS (6-fold) RNAs occurred 6 hr after DEX administration in PBS-pretreated animals. Pretreatment with DMN caused at least a 42% inhibition of DEX-induced RNA accumulation at every time point examined, with greater than 90% inhibition occurring when the genes were maximally induced at 6 hr. This inhibition was not due to any alterations of the glucocorticoid receptors as DMN had no effect on the binding affinity or amounts of glucocorticoid receptors present in rat hepatic cytosols. These results suggest that chemical carcinogens such as DMN may affect normal gene function in vivo by inhibiting the cellular response to hormone receptors mediating differentiation-associated, specialized cell functions.

Mechanisms of chromium carcinogenicity and toxicity

Chromium, like many transition metal elements, is essential to life at low concentrations yet toxic to many systems at higher concentrations. In addition to the overt symptoms of acute chromium toxicity, delayed manifestations of chromium exposure become apparent by subsequent increases in the incidence of various human cancers. Chromium is widely used in numerous industrial processes, and as a result is a contaminant of many environmental systems. Chromium, in its myriad chemical forms and oxidation states, has been well studied in terms of its general chemistry and its interactions with biological molecules. However, the precise mechanisms by which chromium is both an essential metal and a carcinogen are not yet fully clear. The following review does not seek to embellish upon the proposed mechanisms of the toxic and carcinogenic actions of chromium, but rather provides a comprehensive review of these theories. The chemical nature of chromium compounds and how these properties impact upon the interactions of chromium with cellular and genetic targets, including animal and human hosts, are discussed.

Preferential alteration of inducible gene expression in vivo by carcinogens that induce bulky DNA lesions

Our laboratory is interested in whether chemical carcinogen-induced DNA damage is nonrandomly distributed in the genome, i.e., "targeted," at the level of individual genes. To examine this, we have been investigating whether carcinogen treatment in vivo differentially alters the expression of specific genes. In this study, we examined the effects of four model carcinogens that induce bulky lesions in DNA--benzo[a]pyrene (B[a]P), aflatoxin B1 (AFB1), 7,12-dimethylbenz[a]anthracene (DMBA), and 2-acetylaminofluorene (AAF)--on the steady-state mRNA expression of several constitutive and drug-inducible genes in vivo. We specifically tested the hypothesis that carcinogen-induced DNA damage is preferentially targeted to inducible genes relative to constitutively expressed genes using the chick embryo as a simple in vivo test system. In summary, the four carcinogens had no effect on the steady-state mRNA expression of constitutively expressed beta-actin, transferrin, or albumin genes over a 24-h period after a single dose of each carcinogen. In contrast, each of these same treatments significantly altered the mRNA expression of two glutethimide-inducible genes, ALA synthase and CYP2H1. Both the basal expression of these genes and their drug-inducible expression was altered. B[a]P and AFB1 had similar effects on expression of the two inducible genes and caused similar levels of covalent adducts in total DNA, even though the administered doses differed by 30-fold. B[a]P binding to DNA, and the basal expression of CYP2H1 were similar in liver and lung. However, B[a]P significantly altered basal CYP2H1 mRNA expression in liver, a tissue in which this gene is highly inducible by glutethimide, and had no effect on basal CYP2H1 mRNA expression in lung, a tissue in which this gene is not drug-inducible. These data support the hypothesis that inducible gene expression is a target for carcinogen-induced DNA damage in vivo.

Inhibition of protein synthesis increases the transcription of the phenobarbital-inducible CYP2H1 and CYP2H2 genes in chick embryo hepatocytes

The mechanism by which phenobarbital and similar compounds regulate gene expression has remained elusive for many years despite intense investigation. We had previously reported that the mRNA expression for the phenobarbital-inducible CYP2H1 gene was increased by cycloheximide treatment as rapidly and to a similar extent as by the phenobarbital-type drugs glutethimide and 2-propyl-2-isopropylacetamide (PIA), in primary cultures of chick embryo hepatocytes or in chick embryo liver in vivo (J. W. Hamilton, W. J. Bement, P. R. Sinclair, J. F. Sinclair, and K. E. Wetterhahn, 1988, Biochem. J. 255, 267-275). To examine the mechanism of this induction further, we determined the effects of various structurally related and unrelated inhibitors of protein synthesis on CYP2H1 expression in cultured chick embryo hepatocytes. Cycloheximide increased the transcription rate of the CYP2H1/2 genes to a similar extent as did PIA, and had little or no effect on CYP2H1 mRNA half-life. A number of other protein synthesis inhibitors, including streptovitacin, acetoxycycloheximide, pactamycin, and ricin, all increased CYP2H1 mRNA expression to a similar extent. The dose responses for induction of CYP2H1 mRNA and inhibition of protein synthesis by these agents were closely correlated. There was no relationship between the effectiveness of these agents to induce CYP2H1 mRNA expression and their structures or lipophilicity. Cycloheximide acetate required deesterification to cycloheximide for both inhibition of protein synthesis and induction of CYP2H1 mRNA. These results suggest that a labile negative regulatory protein is involved in CYP2H1/2 gene expression. It is also possible that this factor is involved in regulating the phenobarbital response of CYP2H1/2.