Basic mechanics of DNA methylation and the unique landscape of the DNA methylome in metal-induced carcinogenesis

Mid-region parathyroid hormone-related protein is a genome-wide chromatin-binding factor that promotes growth and differentiation of HB2 epithelial cells from the human breast

Human parathyroid hormone-related protein (PTHrP) is a polyhormone that undergoes proteolytic cleavage producing smaller peptides which exert diversified biological effects. PTHrP signalization is prominently involved in breast development and physiology, but the studies have been focused onto either N-terminal species or full-length protein introduced by gene transfer techniques. Our present work investigates for the first time the effect of the mid-region PTHrP secretory form, that is, the fragment [38-94], on HB2 non-tumoral breast epithelial cells. We examined viability/proliferation of cells grown in PTHrP-containing media supplemented with different serum concentration and on different substrates, extending our investigation to check whether (a) by analogy with MDA-MB231 cells, also HB2 cell chromatin possesses genome-wide binding sites for mid-region PTHrP, and (b) the peptide is endowed with modulating properties toward the expression of proliferation/differentiation signatures by HB2 cells. Our results indicate that mid-region PTHrP acts as a cell growth/differentiation stimulator in routine and "nutrient stress" culture conditions, accordingly reprogramming gene expression, and is able to bind to cytogenetic preparations from HB2 cells. This supports the concept that the physiological mechanisms involving PTHrP during breast development may include mature processed forms of the protein different from the N-terminal fragment. © 2018 BioFactors, 45(2):279-288, 2019.

Lead induces the up-regulation of the protein arginine methyltransferase 5 possibly by its promoter demethylation

The studies on lead (Pb) exposure linking to epigenetic modulations are caused by its differential actions on global DNA methylation and histone modifications. These epigenetic changes may result in increased accessibility of the transcription factors to promoter DNA-binding elements leading to activation and expression of the gene. The protein arginine methyltransferase 5 (PRMT5) and its partner methylosome protein 50 (MEP50) together catalyze the mono- and symmetric dimethylation of arginine residues in many histone and non-histone protein substrates. Moreover, it is overexpressed in many forms of cancer. In the present study, the effects of Pb on the PRMT5 and MEP50 expression and formation of the symmetrically dimethylated arginine (SDMA), the catalytic product of the PRMT5-MEP50 complex were analyzed in vitro after exposing the A549 and MCF-7 cells. The results show that exposure to 0.1 and 1 µM of Pb strongly enhanced the expression of both PRMT5 and MEP50 transcript and protein leading to increased SDMA levels globally with H4R3 being increasingly symmetrically dimethylated in a dose-dependent manner after 48 h of Pb exposure in both cell types. The methylation-specific PCR also revealed that the CpG island present on the PRMT5 promoter proximal region was increasingly demethylated as the dose of Pb increased in a 48-h exposure window in both cells, with MCF-7 being more responsive to Pb-mediated PRMT5 promoter demethylation. The bisulfite sequencing confirmed this effect. The findings therefore indicate that Pb exposure increasing the PRMT5 expression might be one of the contributing epigenetic factors in the lead-mediated disease processes as PRMT5 has a versatile role in cellular functions and oncogenesis.

Upregulation of histone-lysine methyltransferases plays a causal role in hexavalent chromium-induced cancer stem cell-like property and cell transformation

While hexavalent chromium [Cr(VI)] is generally considered as a genotoxic environmental carcinogen, studies showed that Cr(VI) exposure also causes epigenetic changes. However, whether Cr(VI)-caused epigenetic dysregulations plays an important role in Cr(VI) carcinogenicity remain largely unknown. The aim of this study was to determine if chronic low dose Cr(VI) exposure causes epigenetic changes, the underlying mechanism and whether chronic low dose Cr(VI) exposure-caused epigenetic dysregulation contributes causally to Cr(VI)-induced cancer stem cell (CSC)-like property and cell transformation. Two immortalized human bronchial epithelial cell lines (BEAS-2B and 16HBE) were exposed to 0.25 μM of K2Cr2O7 for 20 and 40 weeks to induce cell transformation, respectively. Cr(VI)-induced epigenetic changes were examined in Cr(VI)-transformed cells and Cr(VI) exposure-caused human lung cancer tissues. Pharmacological inhibitors and gene knockdown experiments were used to determine the role of epigenetic dysregulation in Cr(VI) carcinogenicity. We found that chronic Cr(VI) exposure causes epigenetic dysregulation as evidenced by the increased levels of histone H3 repressive methylation marks (H3K9me2 and H3K27me3) and the related histone-lysing methyltransferases (HMTases). Pharmacological inhibition or knockdown of HMTases reduces H3 repressive methylation marks and malignant phenotypes of Cr(VI)-transformed cells. Moreover, knockdown of HMTases in parental cells significantly reduces chronic Cr(VI) exposure-induced CSC-like property and cell transformation. Further mechanistic study revealed that knockdown of HMTases decreases Cr(VI) exposure-caused DNA damage. Our findings indicate that chronic Cr(VI) exposure increases H3 repressive methylation marks by increasing the related HMTases expression; and that increased expression of HMTases plays a causal role in Cr(VI)-induced CSC-like property and cell transformation.

Environmental Influences on the Epigenome: Exposure- Associated DNA Methylation in Human Populations

DNA methylation is the most well studied of the epigenetic regulators in relation to environmental exposures. To date, numerous studies have detailed the manner by which DNA methylation is influenced by the environment, resulting in altered global and gene-specific DNA methylation. These studies have focused on prenatal, early-life, and adult exposure scenarios. The present review summarizes currently available literature that demonstrates a relationship between DNA methylation and environmental exposures. It includes studies on aflatoxin B₁, air pollution, arsenic, bisphenol A, cadmium, chromium, lead, mercury, polycyclic aromatic hydrocarbons, persistent organic pollutants, tobacco smoke, and nutritional factors. It also addresses gaps in the literature and future directions for research. These gaps include studies of mixtures, sexual dimorphisms with respect to environmentally associated methylation changes, tissue specificity, and temporal stability of the methylation marks.

Endocrine Disruptors and Developmental Origins of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a growing epidemic worldwide, particularly in countries that consume a Western diet, and can lead to life-threatening conditions such as cirrhosis and hepatocellular carcinoma. With increasing prevalence of NAFLD in both children and adults, an understanding of the factors that promote NAFLD development and progression is crucial. Environmental agents, including endocrine-disrupting chemicals (EDCs), which have been linked to other diseases, may play a role in NAFLD development. Increasing evidence supports a developmental origin of liver disease, and early-life exposure to EDCs could represent one risk factor for the development of NAFLD later in life. Rodent studies provide the strongest evidence for this link, but further studies are needed to define whether there is a causal link between early-life EDC exposure and NAFLD development in humans. Elucidating the molecular mechanisms underlying development of NAFLD in the context of developmental EDC exposures may identify biomarkers for people at risk, as well as potential intervention and/or therapeutic opportunities for the disease.

Possible Mechanisms of Mercury Toxicity and Cancer Promotion: Involvement of Gap Junction Intercellular Communications and Inflammatory Cytokines

A number of observations indicate that heavy metals are able to alter cellular metabolic pathways through induction of a prooxidative state. Nevertheless, the outcome of heavy metal-mediated effects in the development of human diseases is debated and needs further insights. Cancer is a well-established DNA mutation-linked disease; however, epigenetic events are perhaps more important and harmful than genetic alterations. Unfortunately, we do not have reliable screening methods to assess/validate the epigenetic (promoter) effects of a physical or a chemical agent. We propose a mechanism of action whereby mercury acts as a possible promoter carcinogen. In the present contribution, we resume our previous studies on mercury tested at concentrations comparable with its occurrence as environmental pollutant. It is shown that Hg(II) elicits a prooxidative state in keratinocytes linked to inhibition of gap junction-mediated intercellular communication and proinflammatory cytokine production. These combined effects may on one hand isolate cells from tissue-specific homeostasis promoting their proliferation and on the other hand tamper the immune system defense/surveillance checkmating the whole organism. Since Hg(II) is not a mutagenic/genotoxic compound directly affecting gene expression, in a broader sense, mercury might be an example of an epigenetic tumor promoter or, further expanding this concept, a “metagenetic” effector.

Histone variants in environmental-stress-induced DNA damage repair

Environmental stress such as genotoxic agents can cause DNA damage either indirectly through the generation of reactive oxygen species or directly by interactions with the DNA molecule. Damage to the genetic material may cause mutations and ultimately cancer. Genotoxic mutation can be prevented either by apoptosis or DNA repair. In response to DNA damage, cells have evolved DNA damage responses (DDR) to detect, signal, and repair DNA lesions. Epigenetic mechanisms play critically important roles in DDR, which requires changes in chromatin structure and dynamics to modulate DNA accessibility. Incorporation of histone variants into chromatin is considered as an epigenetic mechanism. Canonical histones can be replaced with variant histones that change chromatin structure, stability, and dynamics. Recent studies have demonstrated involvement of nearly all histone variants in environmental-stress-induced DNA damage repair through various mechanisms, including affecting nucleosome dynamics, carrying variant-specific modification, promoting transcriptional competence or silencing, mediating rearrangement of chromosomes, attracting specific repair proteins, among others. In this review, we will focus on the role of histone variants in DNA damage repair after exposure to environmental genotoxic agents. Understanding the mechanisms regulating environmental exposure-induced epigenetic changes, including replacement of canonical histones with histone variants, will promote the development of strategies to prevent or reverse these changes.

Metformin alleviates nickel-induced autophagy and apoptosis via inhibition of hexokinase-2, activating lipocalin-2, in human bronchial epithelial cells

Autophagy is an intracellular recycling and degradation process for regulating tumor progression, survival and drug resistance. Nickel compounds have been identified as human carcinogens. However, the role of nickel-induced autophagy in lung carcinogenesis has not yet been fully elucidated. In this study, we determined that hexokinase 2 (HK2), which phosphorylates glucose and regulates autophagy, is the key mediator in nickel-induced autophagy in lung bronchial epithelial cells. We attempted to investigate the effects of the antidiabetic drug metformin on HK2 expression and lung cancer chemoprevention. Our results showed that metformin decreases nickel-induced autophagy and activation of apoptosis through inhibition of HK2 gene, protein and activity. Furthermore, we demonstrated that lipocalin 2 (LCN2), which is released by neutrophils at sites of infection and inflammation is involved in HK2-driven autophagy pathway. Knockdown of endogenous HK2 and LCN2 by shRNA reduced nickel-elicited autophagy and apoptosis, illustrating that metabolic alteration and inflammatory action are important in nickel-elicited carcinogenesis. We also determined the association between nickel-induced autophagy and apoptosis. Inhibition of nickel-induced autophagy abolished apoptotic cell death in chloroquine-treated, shLC3 Beas-2B cells and Atg5^−/− MFFs. From TGCA database and immunohistochemistry analysis, HK2 and LCN2 expression increased in lung squamous cell carcinoma and their related adjacent normal tissues. Taken together, our results demonstrated that metformin alleviates NiCl₂-induced autophagy and apoptosis via HK2-driven LCN2 activation in human bronchial epithelial cells. This novel mechanism provides a strategy for targeting nickel-elicited lung cancer progression, as well as for preventing HK2 cumulative damage triggered by environmental carcinogens.

Genetic and epigenetic determinants of inter-individual variability in responses to toxicants

It is well established that genetic variability has a major impact on susceptibility to common diseases, responses to drugs and toxicants, and influences disease-related outcomes. The appreciation that epigenetic marks also vary across the population is growing with more data becoming available from studies in humans and model organisms. In addition, the links between genetic variability, toxicity outcomes and epigenetics are being actively explored. Recent studies demonstrate that gene-by-environment interactions involve both chromatin states and transcriptional regulation, and that epigenetics provides important mechanistic clues to connect expression-related quantitative trait loci (QTL) and disease outcomes. However, studies of Gene×Environment×Epigenetics further extend the complexity of the experimental designs and create a challenge for selecting the most informative epigenetic readouts that can be feasibly performed to interrogate multiple individuals, exposures, tissue types and toxicity phenotypes. We propose that among the many possible epigenetic experimental methodologies, assessment of chromatin accessibility coupled with total RNA levels provides a cost-effective and comprehensive option to sufficiently characterize the complexity of epigenetic and regulatory activity in the context of understanding the inter-individual variability in responses to toxicants.

Effects of chronic cobalt and chromium exposure after metal-on-metal hip resurfacing: An epigenome-wide association pilot study

Metal-on-metal (MOM) hip resurfacing has recently been a popular prosthesis choice for the treatment of symptomatic arthritis, but results in the release of cobalt and chromium ions into the circulation that can be associated with adverse clinical effects. The mechanism underlying these effects remains unclear. While chromosomal aneuploidy and translocations are associated with this exposure, the presence of subtle structural epigenetic modifications in patients with MOM joint replacements remains unexplored. Consequently, we analyzed whole blood DNA methylation in 34 OA patients with MOM hip resurfacing (MOM HR) compared to 34 OA patients with non-MOM total hip replacements (non-MOM THR), using the genome-wide Illumina HumanMethylation 450k BeadChip. No probes showed differential methylation significant at 5% false-discovery rate (FDR). We also tested association of probe methylation levels with blood chromium and cobalt levels directly; there were no significant associations at 5% FDR. Finally, we used the "epigenetic clock" to compare estimated to actual age at sample for all individuals. We found no significant difference between MOM HR and non-MOM THR, and no correlation of age acceleration with blood metal levels. Our results suggest the absence of large methylation differences systemically following metal exposure, however, larger sample sizes will be required to identify potential small effects. Any DNA methylation changes that may occur in the local periprosthetic tissues remain to be elucidated. © 2017 The Authors. Orthopaedic Research Society. Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 35:2323-2328, 2017.

Electrozymographic evaluation of the attenuation of arsenic induced degradation of hepatic SOD, catalase in an in vitro assay system by pectic polysaccharides of Momordica charantia in combination with curcumin

Momordica charantia (MC) fruit known as bitter gourd, is of potential nutritional and medicinal value. The objectives of the present in vitro study were to evaluate the efficacy of bioactive pectic polysaccharides (CCPS) of MC along with another well-known bioactive compound curcumin in the abrogation of hepatocellular oxidative stress persuaded by sodium arsenite. Electrozymographic method was developed for the assessment of superoxide dismutase (SOD) and catalase activities of liver tissues maintained under an in vitro system. A significant association of CCPS of MC in combination with curcumin was found in the alleviation of oxidative stress induced by sodium arsenite in liver slice. Generated data pointed out that CCPS of MC and curcumin separately or in combination can offer significant protection against alterations in malondialdehyde (MDA), conjugated diene (CD) and antioxidative defense (SOD, CAT) markers. Furthermore, results of hepatic cell DNA degradation strongly supported that both these co-administrations have efficacy in preventing cellular damage. This is the first information of extracted polysaccharides from MC preventing arsenic induced damage in a liver slice of rat.

The environment and male reproduction: The effect of cadmium exposure on reproductive function and its implication in fertility

Cadmium is an environmental pollutant known as endocrine disruptor. Testis is particularly susceptible to cadmium, and testis injury occurs at high but even low levels of exposure. Cadmium reproductive toxicity is mediated by multiple mechanisms, including structural damage to testis vasculature and blood-testis barrier, inflammation, cytotoxicity on Sertoli and Leydig cells, oxidative stress mainly by means of mimicry and interference with essential ions, apoptosis, interference with selected signaling pathways and epigenetic regulation of genes involved in the regulation of reproductive function, and disturbance of the hypothalamus-pituitary-gonadal axis. The current review outlines epidemiological observational findings from environmental and occupational exposure in humans, and reports experimental studies in humans and animals. Lastly, a focus on the pathogenetic mechanisms of cadmium toxicity and on the specific mechanisms of cadmium sensitivity and resistance, particularly assessed in animal models, is included. Despite convincing experimental findings in animals and supporting evidences in humans identifying cadmium as reproductive toxicant, observational findings are controversial, suffering from heterogeneity of study design and pattern of exposure, and from co-exposure to multiple pollutants.

Toxicity of cadmium and its health risks from leafy vegetable consumption

Cadmium (Cd) is a highly toxic heavy metal and has spread widely in the environment in recent decades. This review summarizes current knowledge about Cd contamination of leafy vegetables, its toxicity, exposure, health risks, and approaches to reducing its toxicity in humans. Leafy vegetable consumption has been identified as a dominant exposure pathway of Cd in the human body. An overview of Cd pollution in leafy vegetables as well as the main sources of Cd is given. Notable estimated daily intakes and health risks of Cd exposure through vegetable consumption for humans are revealed in occupational exposure areas and even in some reference areas. Vegetable consumption is one of the most significant sources of exposure to Cd, particularly in occupational exposure regions. Therefore, numerous approaches have been developed to minimize the accumulation of Cd in leafy vegetables, among which the breeding of Cd pollution-safe cultivars is one of the most effective tools. Furthermore, dietary supplements from leafy vegetables perform positive roles in alleviating Cd toxicity in humans with regard to the effects of essential mineral elements, vitamins and phytochemicals taken into the human body via leafy vegetable consumption.

Nickel and cadmium-induced SLBP depletion: A potential pathway to metal mediated cellular transformation

Both nickel and cadmium compounds have been established as group I carcinogens for several decades. Despite over-whelming evidence of these compounds' carcinogenicity in humans, the specific underlying molecular mechanisms that govern metal induced cellular transformation remain unclear. In this study, we found that there were slightly different effects on decreased SLBP mRNA and protein as well as increased polyA H3.1 in our nickel exposed cells. This suggested that nickel and arsenic have similar effects on canonical histone mRNA transcription and translation. We also saw that the depletion of SLBP protein was reversed by inhibiting the proteosome. Finally, we showed that inhibiting the SLBP mRNA and protein levels were rescued by epigenetic modifiers suggesting that nickel's effects on SLBP may be mediated via epigenetic mechanisms. Taken together these results suggest a similar mechanism by which both arsenic and nickel may exert their carcinogenic effects.

Basic studies on epigenetic carcinogenesis of low-dose exposure to 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo) in vitro

1-Trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo) has been widely studied as a neurotoxic substance, however, only few reports have explored its effect on carcinogenicity. Since the aberrant modification of DNA methylation occurs very early in almost all human cancers, the focus of this study is to assess the carcinogenicity of TaClo by characterizing alterations of the epigenetic state, specifically, DNA methylation, upon exposure to TaClo in a HEK 293 model cell line. Our results suggest that TaClo could induce global DNA hypomethylation and transcriptional repression of critical tumor suppressor genes by increasing their promoter methylation. Enhanced cell proliferation, migration and anchorage independent growth were observed in cells exposed to TaClo. Our study highlights the epigenetic toxicity of TaClo, which contributes to its carcinogenicity by altering the DNA methylation status.

Role of connexin 43 in cadmium-induced proliferation of human prostate epithelial cells

Connexins (Cxs), the subunits of gap junction channels, are involved in many physiological processes. Aberrant control of Cxs and gap junction intercellular communication may contribute to many diseases, including the promotion of cancer. Cd exposure is associated with increased risk of human prostate cancer and benign prostatic hyperplasia. The roles of Cxs in the effects of Cd on the prostate have, however, not been reported previously. In this study, the human prostate epithelial cell line RWPE-1 was exposed to Cd. A low dose of Cd stimulated cell proliferation along with a lower degree of gap junction intercellular communication and an elevated level of the protein Cx43. Cd exposure increased the levels of intracellular Ca²⁺ and phosphorylated Cx43 at the Ser368 site. Knockdown of Cx43 using siRNA blocked Cd-induced proliferation and interfered with the Cd-induced changes in the protein levels of cyclin D1, cyclin B1, p27^Kip1 (p27) and p21^Waf1/Cip1 (p21). The increase in Cx43 expression induced by Cd was presumably mediated by the androgen receptor, because it was abolished upon treatment with the androgen receptor antagonist, flutamide. Thus, a low dose of Cd promotes cell proliferation in RWPE-1, possibly mediated by Cx43 expression through an effect on cell cycle-associated proteins. Cx43 might be a target for prostatic diseases associated with Cd exposure. Copyright © 2017 John Wiley & Sons, Ltd.

Prenatal airborne polycyclic aromatic hydrocarbon exposure, LINE1 methylation and child development in a Chinese cohort

BACKGROUND

Polycyclic aromatic hydrocarbons (PAH) are carcinogenic, neurotoxic environmental pollutants generated during incomplete combustion of fossil fuel and other organic material. PAH exposure has been associated with adverse fetal development and epigenetic alterations in cord blood. Several molecular epidemiology studies have established PAH-DNA adducts as biomarkers of PAH exposure.

OBJECTIVES

We investigated the relationship between LINE1 DNA methylation and PAH-DNA adduct levels in cord blood, and with neurodevelopmental outcomes.

METHODS

In Tongliang County, China, the current study enrolled two population-based cohorts of nonsmoking pregnant women before (2002) and after (2005) the closure of a local coal-fired power plant in May 2004. We analyzed cord blood samples collected from mothers in the two cohorts (n=110 from 2002 cohort and n=107 from 2005 cohort) for PAH-DNA adducts and genomic LINE1 DNA methylation. Neurodevelopmental data on children were collected using the Gesell Developmental Scales (GDS) at age 2 and using the Wechsler Intelligence Scale for Children (WISC) at age 5.

RESULTS

A significant inverse relationship was observed between PAH-DNA adducts and LINE1 DNA methylation (β=-0.010, p<0.038). A significant, positive association between LINE1 methylation and scores on WISC full scale and verbal (β=85.31, p<0.005; β=94.36, p<0.003) but not on the GDS. Mediation analysis did not find LINE1 to be a direct mediator between PAH-DNA adducts and IQ score.

CONCLUSION

LINE1 methylation in cord blood DNA was a positive predictor of IQ at age 5 and was decreased at higher levels of prenatal PAH exposure measured by PAH-DNA adducts in cord blood. However, the adverse effects of prenatal exposure to PAH on IQ scores did not appear to be directly mediated by altered LINE1 methylation.

Relationships among DNA hypomethylation, Cd, and Pb exposure and risk of cigarette smoking-related urothelial carcinoma

Cigarette smoking and environmental exposure to heavy metals are important global health issues, especially for urothelial carcinoma (UC). However, the effects of cadmium and lead exposure, as well as the levels of DNA hypomethylation, on UC risk are limited. We evaluated the possible exposure sources of Cd and Pb and the relationship among DNA hypomethylation, urinary Cd and Pb levels, and UC risk. We recruited 209 patients with UC and 417 control patients for a hospital-based case-control study between June 2011 and August 2014. We collected environmental exposure-related information with questionnaires. Blood and urine samples were analyzed to measure the Cd and Pb exposure and 5-methyl-2'-deoxycytidine levels as a proxy for DNA methylation. Multivariate logistic regression and 95% confidence intervals were applied to estimate the risk for UC. Study participants with high Cd and Pb exposure in blood or urine had significantly increased risk of UC, especially among the smokers. After adjusting for age and gender, the possible connections of individual cumulative cigarette smoking or herb medicine exposure with the increased levels of Cd and Pb were observed in the controls. Participants with 8.66%-12.39% of DNA hypomethylation had significantly increased risk of UC compared with those with ≥12.39% of DNA hypomethylation. Environmental factors including cigarette smoking and herb medicine may contribute to the internal dose of heavy metals levels. Repeat measurements of heavy metals with different study design, detailed dietary information, and types of herb medicine should be recommended for exploring UC carcinogenesis in future studies.

Differential crosstalk between global DNA methylation and metabolomics associated with cell type specific stress response by pristine and functionalized MWCNT

The present study endeavored to evaluate the comprehensive mechanisms of MWCNT-induced toxicity with particular emphasis on understanding cell specificity in relation to surface functionalization of MWCNT. Following treatment with differentially functionalized (hydroxylation/carboxylation) MWCNT on human bronchial epithelial (BEAS-2B) and human hepatoma (HepG2) cell lines, intracellular uptake, various toxicological end points, global metabolomics profiling and DNA methylation were evaluated. Herein, the comparative in vitro studies ascertained that surface functionalization diminished the toxic potentiality of MWCNT in respect of their pristine counterpart. The surface enhanced Raman scattering with dark-field microscopy attested the intracellular uptake of functionalized-MWCNT, but not the pristine one. The MWCNT's exposure caused alterations in stress responses (oxidative stress, inflammation, profibrosis, DNA damage-repair), differential mode of gene expressions, global metabolomics and DNA methylation status (DNMT3B dependent hypo-methylation in BEAS-2B cells and hyper-methylation in HepG2 cells) in a cell type specific and surface functionalization dependent manner. The alterations in particular metabolites (choline, betaine, succinate etc.) and distinct DNA methylation crosstalk patterns are the possible underlying mechanisms of differential mode of gene expressions and cell type specificity of MWCNT. This study provides preliminary evidence of epigenetic modifications and global metabolomics profiling which might be translated for risk assessment of MWCNT.

Trace metals in resuspended fraction of settled bus dust and assessment of non-occupational exposure

Trace metals (TMs) within urban public transportation systems have rarely been studied and information on related health risks is scant. This study measured TM (arsenic, chromium, cadmium, nickel, zinc, copper and lead) concentrations in resuspended fractions of settled bus dust in Harbin, China, and estimated the exposure and health risks. The incremental lifetime cancer risk (ILCR) for commuters was estimated for TM exposures. The average concentration of total TMs was 559μg/g (ranges from 312 to 787) among 45 bus routes in Harbin. The hazard quotient of three selected commuter groups increased in the following order: teenagers<children<adults. Pb had the highest hazard index among six non-carcinogenic risk TMs. The ILCR for passengers was estimated for TM exposure and was shown to be acceptable (1.00E-07-1.00E-05); the sequence of ILCR assessments followed this ascending trend: ingestion>dermal contact>inhalation.

Analysis of the transcriptional regulation of cancer-related genes by aberrant DNA methylation of the cis-regulation sites in the promoter region during hepatocyte carcinogenesis caused by arsenic

Liver is the major organ for arsenic methylation metabolism and may be the potential target of arsenic-induced cancer. In this study, normal human liver cell was treated with arsenic trioxide, and detected using DNA methylation microarray. Some oncogenes, tumor suppressor genes, transcription factors (TF), and tumor-associated genes (TAG) that have aberrant DNA methylation have been identified. However, simple functional studies of genes adjacent to aberrant methylation sites cannot well reflect the regulatory relationship between DNA methylation and gene transcription during the pathogenesis of arsenic-induced liver cancer, whereas a further analysis of the cis-regulatory elements and their trans-acting factors adjacent to DNA methylation can more precisely reflect the relationship between them. MYC and MAX (MYC associated factor X) were found to participating cell cycle through a bioinformatics analysis. Additionally, it was found that the hypomethylation of cis-regulatory sites in the MYC promoter region and the hypermethylation of cis-regulatory sites in the MAX promoter region result in the up-regulation of MYC mRNA expression and the down-regulation of MAX mRNA, which increased the hepatocyte carcinogenesis tendency.

Molecular Mechanisms of Malignant Transformation by Low Dose Cadmium in Normal Human Bronchial Epithelial Cells

Cadmium is a carcinogenic metal, the mechanisms of which are not fully understood. In this study, human bronchial epithelial cells were transformed with sub-toxic doses of cadmium (0.01, 0.05, and 0.1 μM) and transformed clones were characterized for gene expression changes using RNA-seq, as well as other molecular measurements. 440 genes were upregulated and 47 genes were downregulated in cadmium clones relative to control clones over 1.25-fold. Upregulated genes were associated mostly with gene ontology terms related to embryonic development, immune response, and cell movement, while downregulated genes were associated with RNA metabolism and regulation of transcription. Several embryonic genes were upregulated, including the transcription regulator SATB2. SATB2 is critical for normal skeletal development and has roles in gene expression regulation and chromatin remodeling. Small hairpin RNA knockdown of SATB2 significantly inhibited growth in soft agar, indicating its potential as a driver of metal-induced carcinogenesis. An increase in oxidative stress and autophagy was observed in cadmium clones. In addition, the DNA repair protein O⁶-methylguanine-DNA-methyltransferase was depleted by transformation with cadmium. MGMT loss caused significant decrease in cell viability after treatment with the alkylating agent temozolomide, demonstrating diminished capacity to repair such damage. Results reveal various mechanisms of cadmium-induced malignant transformation in BEAS-2B cells including upregulation of SATB2, downregulation of MGMT, and increased oxidative stress.

Differential genotoxic and epigenotoxic effects of graphene family nanomaterials (GFNs) in human bronchial epithelial cells

The widespread applications of graphene family nanomaterials (GFNs) raised the considerable concern over human health and environment. The cyto-genotoxic potentiality of GFNs has attracted much more attention, albeit the potential effects on the cellular epigenome remain largely unknown. The effects of GFNs on cellular genome were evaluated with single and double stranded DNA damage and DNA repair gene expressions while the effects on epigenome was accomplished by addressing the global DNA methylation and expression of DNA methylation machineries at non-cytotoxic to moderately cytotoxic doses in in vitro system. We used five different representatives of GFNs-pristine (GNP-Prist), carboxylated (GNP-COOH) and aminated (GNP-NH2) graphene nanoplatelets as well as single layer (SLGO) and few layer (FLGO) graphene oxide. The order of single stranded DNA damage was observed as GNP-Prist ≥ GNP-COOH>GNP-NH2≥FLGO>SLGO at 10mg/L and marked dose dependency was found in SLGO. The GFNs possibly caused genotoxicity by affecting nucleotide excision repair and non-homologus end joining repair systems. Besides, dose dependent increase in global DNA methylation (hypermethylation) were observed in SLGO/FLGO exposure and conversely, GNPs treatment caused hypomethylation following the order as GNP-COOH>GNP-NH2 ≥ GNP-Prist. The decrements of DNA methyltransferase (DNMT3B gene) and methyl-CpG binding domain protein (MBD1) genes were probably the cause of global hypomethylation induced by GNPs. Conversely, the de novo methylation through the up-regulation of DNMT3B and MBD1 genes gave rise to the global DNA hypermethylation in SLGO/FLGO treated cells. In general, the GFNs induced genotoxicity and alterations of global DNA methylation exhibited compounds type specificity with differential physico-chemical properties. Taken together, our study suggests that the GFNs could cause more subtle changes in gene expression programming by modulating DNA methylation status and this information would be helpful for their prospective use in biomedical field.

Regulation of iron homeostasis by the p53-ISCU pathway

Accumulation of iron in tissues increases the risk of cancer, but iron regulatory mechanisms in cancer tissues are largely unknown. Here, we report that p53 regulates iron metabolism through the transcriptional regulation of ISCU (iron-sulfur cluster assembly enzyme), which encodes a scaffold protein that plays a critical role in Fe-S cluster biogenesis. p53 activation induced ISCU expression through binding to an intronic p53-binding site. Knockdown of ISCU enhanced the binding of iron regulatory protein 1 (IRP1), a cytosolic Fe-S protein, to an iron-responsive element in the 5′ UTR of ferritin heavy polypeptide 1 (FTH1) mRNA and subsequently reduced the translation of FTH1, a major iron storage protein. In addition, in response to DNA damage, p53 induced FTH1 and suppressed transferrin receptor, which regulates iron entry into cells. HCT116 p53^+/+ cells were resistant to iron accumulation, but HCT116 p53^−/− cells accumulated intracellular iron after DNA damage. Moreover, excess dietary iron caused significant elevation of serum iron levels in p53^−/− mice. ISCU expression was decreased in the majority of human liver cancer tissues, and its reduced expression was significantly associated with p53 mutation. Our finding revealed a novel role of the p53-ISCU pathway in the maintenance of iron homeostasis in hepatocellular carcinogenesis.

Tungsten-induced carcinogenesis in human bronchial epithelial cells

Metals such as arsenic, cadmium, beryllium, and nickel are known human carcinogens; however, other transition metals, such as tungsten (W), remain relatively uninvestigated with regard to their potential carcinogenic activity. Tungsten production for industrial and military applications has almost doubled over the past decade and continues to increase. Here, for the first time, we demonstrate tungsten's ability to induce carcinogenic related endpoints including cell transformation, increased migration, xenograft growth in nude mice, and the activation of multiple cancer-related pathways in transformed clones as determined by RNA sequencing. Human bronchial epithelial cell line (Beas-2B) exposed to tungsten developed carcinogenic properties. In a soft agar assay, tungsten-treated cells formed more colonies than controls and the tungsten-transformed clones formed tumors in nude mice. RNA-sequencing data revealed that the tungsten-transformed clones altered the expression of many cancer-associated genes when compared to control clones. Genes involved in lung cancer, leukemia, and general cancer genes were deregulated by tungsten. Taken together, our data show the carcinogenic potential of tungsten. Further tests are needed, including in vivo and human studies, in order to validate tungsten as a carcinogen to humans.

Oral Chromium Exposure and Toxicity

Hexavalent chromium [Cr(VI)] is a known carcinogen when inhaled. However, inhalational exposure to Cr(VI) affects only a small portion of the population, mainly by occupational exposures. In contrast, oral exposure to Cr(VI) is widespread and affects many people throughout the globe. In 2008, the National Toxicology Program (NTP) released a 2-year study demonstrating that ingested Cr(VI) was carcinogenic in rats and mice. The effects of Cr(VI) oral exposure are mitigated by reduction in the gut; however, a portion evades the reductive detoxification and reaches target tissues. Once Cr(VI) enters the cell, it ultimately gets reduced to Cr(III), which mediates its toxicity via induction of oxidative stress during the reduction while Cr intermediates react with protein and DNA. Cr(III) can form adducts with DNA that may lead to mutations. This review will discuss the potential adverse effects of oral exposure to Cr(VI) by presenting up-to-date human and animal studies, examining the underlying mechanisms that mediate Cr(VI) toxicity, as well as highlighting opportunities for future research.

Gene expression and pathway analysis of human hepatocellular carcinoma cells treated with cadmium

Cadmium (Cd) is a toxic and carcinogenic metal naturally occurring in the Earth's crust. A common route of human exposure is via diet and cadmium accumulates in the liver. The effects of Cd exposure on gene expression in human hepatocellular carcinoma (HepG2) cells were examined in this study. HepG2 cells were acutely-treated with 0.1, 0.5, or 1.0 μM Cd for 24h; or chronically-treated with 0.01, 0.05, or 0.1 μM Cd for three weeks and gene expression analysis was performed using Affymetrix GeneChip® Human Gene 1.0 ST Arrays. Acute and chronic exposures significantly altered the expression of 333 and 181 genes, respectively. The genes most upregulated by acute exposure included several metallothioneins. Downregulated genes included the monooxygenase CYP3A7, involved in drug and lipid metabolism. In contrast, CYP3A7 was upregulated by chronic Cd exposure, as was DNAJB9, an anti-apoptotic J protein. Genes downregulated following chronic exposure included the transcriptional regulator early growth response protein 1. Ingenuity Pathway Analysis revealed that the top networks altered by acute exposure were lipid metabolism, small molecule biosynthesis, cell morphology, organization, and development; while top networks altered by chronic exposure were organ morphology, cell cycle, cell signaling, and renal and urological diseases/cancer. Many of the dysregulated genes play important roles in cellular growth, proliferation, and apoptosis, and may be involved in carcinogenesis. In addition to gene expression changes, HepG2 cells treated with cadmium for 24h indicated a reduction in global levels of histone methylation and acetylation that persisted 72 h post-treatment.

Dynamic changes of global DNA methylation and hypermethylation of cell adhesion-related genes in rat kidneys in response to ochratoxin A

Ochratoxin A (OTA), which is found in a variety of food products, is associated with the development of nephrotoxicity and carcinogenicity in rats and has raised public health concerns. A previous study in our laboratory indicated that OTA exposure induced cytotoxicity by decreasing global DNA methylation in vitro. However, the relationship between OTA-induced nephrotoxicity and DNA methylation changes in vivo remains unclear. The object of this study was to investigate whether OTA can change global DNA methylation or alter the expression of several critical tumour-related genes by inducing methylation modifications before carcinogenesis. We focused on the mechanism of action of OTA in regard to DNA methylation, including the expression of DNA methyltransferases and the regulation of specific cell signalling pathways. Dynamic and dose-dependent changes of global DNA methylation were observed during OTA-induced nephrotoxicity and probably associated with the expression of DNA methyltransferase 1. 13-week exposure of OTA caused hypermethylation in the promoters of critical cell adhesion-related genes, E-cadherin and N-cadherin, leading to reduction of the corresponding mRNA expression, accompanied by transcriptional activation of the Wnt and PI3K/AKT pathways. These findings suggested that long-term OTA exposure could disrupt DNA methylation profile, which might be one of the possible mechanisms of OTA-induced nephrotoxicity.

Epimutagenesis: A prospective mechanism to remediate arsenic-induced toxicity

Arsenic toxicity is a global issue, addressed by the World Health Organization as one of the major natural calamities faced by humans. More than 137 million individuals in 70 nations are affected by arsenic mainly through drinking water and also through diet. Chronic arsenic exposure leads to various types of patho-physiological end points in humans including cancers. Arsenic, a xenobiotic substance, is biotransformed in the body to its methylated species by using the physiological S-adenosyl methionine (SAM). SAM dictates methylation status of the genome and arsenic metabolism leads to depletion of SAM leading to an epigenetic disequilibrium. Since epigenetics is one of the major phenomenon at the interface between the environment and human health impact, its disequilibrium by arsenic inflicts upon the chromatin compaction, gene expression, genomic stability and a host of biomolecular interactions, the interactome within the cell. Since arsenic is not mutagenic but is carcinogenic in nature, arsenic induced epimutagenesis has come to the forefront since it determines the transcriptional and genomic integrity of the cell. Arsenic toxicity brings forth several pathophysiological manifestations like dermatological non-cancerous, pre-cancerous and cancerous lesions, peripheral neuropathy, DNA damage, respiratory disorders and cancers of several internal organs. Recently, several diseases of similar manifestations have been explained with the relevant epigenetic perspectives regarding the possible molecular mechanism for their onset. Hence, in the current review, we comprehensively try to intercalate the information on arsenic-induced epigenetic alterations of DNA, histones and microRNA so as to understand whether the arsenic-induced toxic manifestations are brought about by the epigenetic changes. We highlight the need to understand the aspect of epimutagenesis and subsequent alterations in the cellular interactome due to arsenic-induced molecular changes, which may be utilized to develop putative therapeutic strategies targeting both oxidative potential and epimutagenesis in humans.

A Potential New Mechanism of Arsenic Carcinogenesis: Depletion of Stem-Loop Binding Protein and Increase in Polyadenylated Canonical Histone H3.1 mRNA

Canonical histones are synthesized with a peak in S-phase, whereas histone variants are formed throughout the cell cycle. Unlike messenger RNA (mRNA) for all other genes with a poly(A) tail, canonical histone mRNAs contain a stem-loop structure at their 3'-ends. This stem-loop structure is the binding site for the stem-loop binding protein (SLBP), a protein involved in canonical histone mRNA processing. Recently, we found that arsenic depletes SLBP by enhancing its proteasomal degradation and epigenetically silencing the promoter of the SLBP gene. The loss of SLBP disrupts histone mRNA processing and induces aberrant polyadenylation of canonical histone H3.1 mRNA. Here, we present new data supporting the idea that the lack of SLBP allows the H3.1 mRNA to be polyadenylated using the downstream poly(A) signal. SLBP was also depleted in arsenic-transformed bronchial epithelial cells (BEAS-2B), which led us to hypothesize the involvement of SLBP and polyadenylated H3.1 mRNA in carcinogenesis. Here, for the first time, we report that overexpression of H3.1 polyadenylated mRNA, and knockdown of SLBP enhances anchorage-independent cell growth. A pcDNA-H3.1 vector with a poly(A) signal sequence was stably transfected into BEAS-2B cells. Polyadenylated H3.1 mRNA and exogenous H3.1 protein levels were significantly increased in cells containing the pcDNA-H3.1 vector. A soft agar assay revealed that cells containing the vector formed significantly higher numbers of colonies compared to wild-type cells. Moreover, small hairpin RNA for SLBP (shSLBP) was used to knockdown the expression of SLBP. Cells stably transfected with the shSLBP vector grew significantly more colonies in soft agar than cells transfected with a control vector. These data suggest that upregulation of polyadenylated H3.1 mRNA holds potential as a mechanism to facilitate carcinogenesis by toxicants such as arsenic that depletes SLBP.

The effect of metallothionein 2A core promoter region single-nucleotide polymorphism on accumulation of toxic metals in sinonasal inverted papilloma tissues

Metallothioneins (MTs) are intracellular thiol-rich heavy metal-binding proteins which join trace metal ions protecting cells against heavy metal toxicity and regulate metal distribution and donation to various enzymes and transcription factors. The goal of this study was to identify the -5 A/G (rs28366003) single-nucleotide polymorphism (SNP) in the core promoter region of the MT2A gene, and to investigate its effect on allele-specific gene expression and Cd, Zn, Cu and Ni content in sinonasal inverted papilloma tissue (IP), with non-cancerous sinonasal mucosa (NCM) as a control. The MT2A promoter region -5 A/G SNP was identified by restriction fragment length polymorphism using 117 IP and 132 NCM. MT2A gene analysis was performed by quantitative real-time PCR. Metal levels were analyzed by flame atomic absorption spectrometry. The frequency of A allele carriage was 99.2% and 100% in IP and NCM, respectively. The G allele carriage was detected in 23.9% of IP and in 12.1% of the NCM samples. As a result, a significant association of -5 A/G SNP in MT2A gene with mRNA expression in both groups was determined. A significant association was identified between the -5 A/G SNP in the MT2A gene with mRNA expression in both groups. A highly significant association was detected between the rs28366003 genotype and Cd and Zn content in IP. Furthermore, significant differences were identified between A/A and A/G genotype with regard to the type of metal contaminant. The Spearman rank correlation results showed the MT2A gene expression and both Cd and Cu levels were negatively correlated. The results obtained in this study suggest that the -5 A/G SNP in the MT2A gene may have an effect on allele-specific gene expression and toxic metal accumulation in sinonasal inverted papilloma.

Assessment on the occupational exposure of urban public bus drivers to bioaccessible trace metals through resuspended fraction of settled bus dust

Limited information is available on the bioaccessible fraction of trace metals in the resuspended fraction of settled bus dust in order to estimate bus drivers' occupational exposure. In this study, 45 resuspended fraction of settled dust samples were collected from gasoline and compressed natural gas (CNG) powered buses and analyzed for trace metals and their fraction concentrations using a three-step sequential extraction procedure. Experimental results showed that zinc (Zn) had the greatest bioaccessible fraction, recorded as an average of 608.53 mg/kg, followed in order of decreasing concentration by 129.80 mg/kg lead (Pb), 56.77 mg/kg copper (Cu), 34.03 mg/kg chromium (Cr), 22.05 mg/kg nickel (Ni), 13.17 mg/kg arsenic (As) and 2.77 mg/kg cadmium (Cd). Among the three settled bus dust exposure pathways, ingestion was the main route. Total exposure hazard index (HIt) for non-carcinogenic effect trace metals was lower than the safety level of 1. The incremental lifetime cancer risk (ILCR) for drivers was estimated for trace metal exposure. Pb and Ni presented relatively high potential risks in the non-carcinogenic and potentially carcinogenic health assessment for all drivers. ILCR was in the range of 1.84E-05 to 7.37E-05 and 1.74E-05 to 6.95E-05 for gasoline and CNG buses, respectively.

Relationship between urinary nickel and methylation of p15, p16 in workers exposed to nickel

OBJECTIVE

The purpose of this study was to investigate the relationship between urinary nickel and methylation of p15, p16 in workers exposed to nickel.

METHODS

In this study, 165 nickel-exposed workers and 67 workers without exposure were recruited. The levels of urinary nickel were analyzed using dimethylglyoxime spectrophotometric method. Methylation-specific polymerase chain reaction was used to detect the methylation of p15 and p16.

RESULTS

The median concentration of urinary nickel in the exposed group (4.58 μg/L) was significantly higher than that in the control group (1.78 μg/L; P < 0.01). The rate of methylation of p15 in the exposed group was significantly higher than that in the control group (P = 0.023). The multiple logistic analysis showed that workers having higher urinary nickel were at the higher risk of methylation of p15 (P = 0.024).

CONCLUSIONS

The levels of urinary nickel were significantly associated with the methylation of p15.

The mechanistic basis of arsenicosis: pathogenesis of skin cancer

Significant amounts of arsenic have been found in the groundwater of many countries including Argentina, Bangladesh, Chile, China, India, Mexico, and the United States with an estimated 200 million people at risk of toxic exposure. Although chronic arsenic poisoning damages many organ systems, it usually first presents in the skin with manifestations including hyperpigmentation, hyperkeratoses, Bowen's disease, squamous cell carcinoma, and basal cell carcinoma. Arsenic promotes oxidative stress by upregulating nicotinamide adenine dinucleotide phosphate oxidase, uncoupling nitric oxide synthase, and by depleting natural antioxidants such as nitric oxide and glutathione in addition to targeting other proteins responsible for the maintenance of redox homeostasis. It causes immune dysfunction and tissue inflammatory responses, which may involve activation of the unfolded protein response signaling pathway. In addition, the dysregulation of other molecular targets such as nuclear factor kappa B, Hippo signaling protein Yap, and the mineral dust-induced proto-oncogene may orchestrate the pathogenesis of arsenic-mediated health effects. The metalloid decreases expression of tumor suppressor molecules and increases expression of pro-inflammatory mitogen-activated protein kinase pathways leading to a tumor-promoting tissue microenvironment. Cooperation of upregulated signal transduction molecules with DNA damage may abrogate apoptosis, promote proliferation, and enhance cell survival. Genomic instability via direct DNA damage and weakening of several cellular DNA repair mechanisms could also be important cancer development mechanisms in arsenic-exposed populations. Thus, arsenic mediates its toxicity by generating oxidative stress, causing immune dysfunction, promoting genotoxicity, hampering DNA repair, and disrupting signal transduction, which may explain the complex disease manifestations seen in arsenicosis.

Arsenic induces polyadenylation of canonical histone mRNA by down-regulating stem-loop-binding protein gene expression

The replication-dependent histone genes are the only metazoan genes whose messenger RNA (mRNA) does not terminate with a poly(A) tail at the 3'-end. Instead, the histone mRNAs display a stem-loop structure at their 3'-end. Stem-loop-binding protein (SLBP) binds the stem-loop and regulates canonical histone mRNA metabolism. Here we report that exposure to arsenic, a carcinogenic metal, decreased cellular levels of SLBP by inducing its proteasomal degradation and inhibiting SLBP transcription via epigenetic mechanisms. Notably, arsenic exposure dramatically increased polyadenylation of canonical histone H3.1 mRNA possibly through down-regulation of SLBP expression. The polyadenylated H3.1 mRNA induced by arsenic was not susceptible to normal degradation that occurs at the end of S phase, resulting in continued presence into mitosis, increased total H3.1 mRNA, and increased H3 protein levels. Excess expression of canonical histones have been shown to increase sensitivity to DNA damage as well as increase the frequency of missing chromosomes and induce genomic instability. Thus, polyadenylation of canonical histone mRNA following arsenic exposure may contribute to arsenic-induced carcinogenesis.

Molecular mechanism of hepatitis B virus X-associated hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is one of the most common malignant diseases and has the fourth highest mortality rate worldwide. Chronic hepatitis B virus (HBV) infection is one of the most important etiological factors for HCC. Current studies show that the hepatitis B virus X (HBX) gene plays an important role in the development of HBV-associated HCC. HBX protein is a multifunctional regulator. Though interacting with different host factors, HBX takes part in many cell physiological activities, such as signal transduction, gene transcription, cell cycle progression, apoptosis and autophagy. This review will discuss the biological role of HBX protein in the development of HCC based on the current state of knowledge on this protein.

Mechanism-based genotoxicity screening of metal oxide nanoparticles using the ToxTracker panel of reporter cell lines

BACKGROUND

The rapid expansion of manufacturing and use of nano-sized materials fuels the demand for fast and reliable assays to identify their potential hazardous properties and underlying mechanisms. The ToxTracker assay is a recently developed mechanism-based reporter assay based on mouse embryonic stem (mES) cells that uses GFP-tagged biomarkers for detection of DNA damage, oxidative stress and general cellular stress upon exposure. Here, we evaluated the ability of the ToxTracker assay to identify the hazardous properties and underlying mechanisms of a panel of metal oxide- and silver nanoparticles (NPs) as well as additional non-metallic materials (diesel, carbon nanotubes and quartz).

METHODS

The metal oxide- and silver nanoparticles were characterized in terms of agglomeration and ion release in cell medium (using photon cross correlation spectroscopy and inductively coupled plasma with optical emission spectroscopy, respectively) as well as acellular ROS production (DCFH-DA assay). Cellular uptake was investigated by means of transmission electron microscopy. GFP reporter induction and cytotoxicity of the NPs was simultaneously determined using flow cytometry, and genotoxicity was further tested using conventional assays (comet assay, γ-H2AX and RAD51 foci formation).

RESULTS

We show that the reporter cells were able to take up nanoparticles and, furthermore, that exposure to CuO, NiO and ZnO nanoparticles as well as to quartz resulted in activation of the oxidative stress reporter, although only at high cytotoxicity for ZnO. NiO NPs activated additionally a p53-associated cellular stress response, indicating additional reactive properties. Conventional assays for genotoxicity assessment confirmed the response observed in the ToxTracker assay. We show for CuO NPs that the induction of oxidative stress is likely the consequence of released Cu ions whereas the effect by NiO was related to the particles per se. The DNA replication stress-induced reporter, which is most strongly associated with carcinogenicity, was not activated by any of the tested nanoparticles.

CONCLUSIONS

We conclude that the ToxTracker reporter system can be used as a rapid mechanism-based tool for the identification of hazardous properties of metal oxide NPs. Furthermore, genotoxicity of metal oxide NPs seems to occur mainly via oxidative stress rather than direct DNA binding with subsequent replication stress.

Chromatin Memory in the Development of Human Cancers

Cancer is a complex disease with acquired genomic and epigenomic alterations that affect cell proliferation, viability and invasiveness. Almost all the epigenetic mechanisms including cytosine methylation and hydroxymethylation, chromatin remodeling and non-coding RNAs have been found associate with carcinogenesis and cancer specific expression profile. Altered histone modification as an epigenetic hallmark is frequently found in tumors. Understanding the epigenetic alterations induced by carcinogens or infectious agents may help us understand early epigenetic changes prior to the development of cancer. In this review, we focus on chromatin remodeling and the associated histone modifiers in the development of cancer; the application of these modifiers as a cancer therapy target in different clinical trial phases is also discussed.

Incorporating epigenetic data into the risk assessment process for the toxic metals arsenic, cadmium, chromium, lead, and mercury: strategies and challenges

Exposure to toxic metals poses a serious human health hazard based on ubiquitous environmental presence, the extent of exposure, and the toxicity and disease states associated with exposure. This global health issue warrants accurate and reliable models derived from the risk assessment process to predict disease risk in populations. There has been considerable interest recently in the impact of environmental toxicants such as toxic metals on the epigenome. Epigenetic modifications are alterations to an individual's genome without a change in the DNA sequence, and include, but are not limited to, three commonly studied alterations: DNA methylation, histone modification, and non-coding RNA expression. Given the role of epigenetic alterations in regulating gene and thus protein expression, there is the potential for the integration of toxic metal-induced epigenetic alterations as informative factors in the risk assessment process. In the present review, epigenetic alterations induced by five high priority toxic metals/metalloids are prioritized for analysis and their possible inclusion into the risk assessment process is discussed.

Evaluating the extent of LINE-1 mobility following exposure to heavy metals in HepG2 cells

The long interspersed elements-1 (LINE1 or L1 retrotransposon) constitute 17% of the human genome and retain mobility properties within the genome. At present, 80-100 human L1 elements are thought to be active in the genome. The mobilization of these active elements may be influenced upon exposure to the heavy metals. In the present study, we evaluated the association of aluminum, lead, and copper exposure with L1 retrotransposition in human hepatocellular carcinoma (HepG2) cell line. An in vitro retrotransposition assay using an enhanced green fluorescent protein (EGFP)-tagged L1RP cassette was established to track EGFP shining as the mark of retrotransposition. Following determination of noncytotoxic concentrations of these metals, pL1RP-EGFP-transfected HepG2 cells were subjected to long-term treatment. Flow cytometry analysis of cells treated with various concentrations of these metals along with quantitative real-time PCR was used to quantify L1 retrotransposition frequencies. Aluminum significantly increased L1 retrotransposition frequency, while no significant association was found concerning lead exposure and L1 retrotransposition. Copper treatment downregulated L1 retrotransposition as a result of EGFP-tagged L1RP expression. Our findings suggest that aluminum might have the potential to cause genomic instability by the enhancement of L1 mobilization. Thus, the risk of induced L1 retrotransposition should be considered during drug safety evaluation and risk assessments of exposure to toxic environmental agents. Further studies are needed for a more robust assay to evaluate any associations between long-term lead exposure and L1 mobility in cell culture assay.

Epigenetic alterations induced by ambient particulate matter in mouse macrophages

Respiratory mortality and morbidity has been associated with exposure to particulate matter (PM). Experimental evidence suggests involvement of cytotoxicity, oxidative stress, and inflammation in the development of PM-associated pathological states; however, the exact mechanisms remain unclear. In the current study, we analyzed short-term epigenetic response to PM10 (particles with aerodynamic diameter less than 10 μm) exposure in mouse ascitic RAW264.7 macrophages (BALB/C Abelson murine leukemia virus-induced tumor). Ambient PM10 was collected using a high volume sampler in Little Rock, AR. Analysis revealed that PM10 was composed mainly of Al and Fe, and the water soluble organic fraction was dominated by aliphatic and carbohydrate fragments and minor quantities of aromatic components. Exposure to PM10 compromised the cellular epigenome at concentrations 10-200 µg/ml. Specifically, epigenetic alterations were evident as changes in the methylation and expression of repetitive element-associated DNA and associated DNA methylation machinery. These results suggest that epigenetic alterations, in concert with cytotoxicity, oxidative stress, and inflammation, might contribute to the pathogenesis of PM-associated respiratory diseases.

10th NTES Conference: Nickel and Arsenic Compounds Alter the Epigenome of Peripheral Blood Mononuclear Cells

The mechanisms that underlie metal carcinogenesis are the subject of intense investigation; however, data from in vitro and in vivo studies are starting to piece together a story that implicates epigenetics as a key player. Data from our lab has shown that nickel compounds inhibit dioxygenase enzymes by displacing iron in the active site. Arsenic is hypothesized to inhibit these enzymes by diminishing ascorbate levels--an important co-factor for dioxygenases. Inhibition of histone demethylase dioxygenases can increase histone methylation levels, which also may affect gene expression. Recently, our lab conducted a series of investigations in human subjects exposed to high levels of nickel or arsenic compounds. Global levels of histone modifications in peripheral blood mononuclear cells (PBMCs) from exposed subjects were compared to low environmentally exposed controls. Results showed that nickel increased H3K4me3 and decreased H3K9me2 globally. Arsenic increased H3K9me2 and decreased H3K9ac globally. Other histone modifications affected by arsenic were sex-dependent. Nickel affected the expression of 2756 genes in human PBMCs and many of the genes were involved in immune and carcinogenic pathways. This review will describe data from our lab that demonstrates for the first time that nickel and arsenic compounds affect global levels of histone modifications and gene expression in exposed human populations.