Histone Methylation in Nickel-Smelting Industrial Workers

Epigenetic toxicity of heavy metals - implications for embryonic stem cells

Exposure to heavy metals, such as cadmium, nickel, mercury, arsenic, lead, and hexavalent chromium has been linked to dysregulated developmental processes, such as impaired stem cell differentiation. Heavy metals are well-known modifiers of the epigenome. Stem and progenitor cells are particularly vulnerable to exposure to potentially toxic metals since these cells rely on epigenetic reprogramming for their proper functioning. Therefore, exposure to metals can impair stem and progenitor cell proliferation, pluripotency, stemness, and differentiation. In this review, we provide a comprehensive summary of current evidence on the epigenetic effects of heavy metals on stem cells, focusing particularly on DNA methylation and histone modifications. Moreover, we explore the underlying mechanisms responsible for these epigenetic changes. By providing an overview of heavy metal exposure-induced alterations to the epigenome, the underlying mechanisms, and the consequences of those alterations on stem cell function, this review provides a foundation for further research in this critical area of overlap between toxicology and developmental biology.

Toxicoepigenetics and Environmental Health: Challenges and Opportunities

The rapidly growing field of toxicoepigenetics seeks to understand how toxicant exposures interact with the epigenome to influence disease risk. Toxicoepigenetics is a promising field of environmental health research, as integrating epigenetics into the field of toxicology will enable a more thorough evaluation of toxicant-induced disease mechanisms as well as the elucidation of the role of the epigenome as a biomarker of exposure and disease and possible mediator of exposure effects. Likewise, toxicoepigenetics will enhance our knowledge of how environmental exposures, lifestyle factors, and diet interact to influence health. Ultimately, an understanding of how the environment impacts the epigenome to cause disease may inform risk assessment, permit noninvasive biomonitoring, and provide potential opportunities for therapeutic intervention. However, the translation of research from this exciting field into benefits for human and animal health presents several challenges and opportunities. Here, we describe four significant areas in which we see opportunity to transform the field and improve human health by reducing the disease burden caused by environmental exposures. These include (1) research into the mechanistic role for epigenetic change in environment-induced disease, (2) understanding key factors influencing vulnerability to the adverse effects of environmental exposures, (3) identifying appropriate biomarkers of environmental exposures and their associated diseases, and (4) determining whether the adverse effects of environment on the epigenome and human health are reversible through pharmacologic, dietary, or behavioral interventions. We then highlight several initiatives currently underway to address these challenges.

Melatonin ameliorates nickel induced autophagy in mouse brain: diminution of oxidative stress

Nickel（Ni) is a neurotoxic environmental pollutant. Oxidative stress is thought to be the main mechanism behind the development of Ni neurotoxicity. Melatonin (Mt) has significant efficacy as an antioxidant. In this paper, we investigated the damage that Ni causes to the autophagy of the nervous system. Furthermore, Mt has can intervene upon the damage caused by Ni, which can protect the nervous system. Herein, we randomly divided 80 8-week-old male wild-type C57BL/6N mice into four groups, including the C group, Ni group, Mt group, and Mt+Ni group. Ni was gavaged at a concentration of 10mg/kg, while was Mt was administered at a concentration of 2mg/kg for 21 days at 0.1ml/10g body weight of the mice. Histopathological and ultrastructural observations demonstrated altered states, such as neuronal atrophy, as well as typical autophagic features in the Ni group. Mt was able to intervene effectively in Ni-induced neurotoxicity. The antioxidant capacity assay also demonstrated that Ni can lead to a large amount of reactive oxygen species (ROS) production within the mouse brain. Furthermore, the same Mt was effective at reducing ROS production. In order to further illustrate this point, we added the broad-spectrum phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 to NS20Y cells. The presence of inhibitors effectively demonstrates that, within the PI3K/AKT/mTOR pathway, autophagy occurs. In conclusion, these data suggest that Ni causes oxidative stress damage and induces autophagy within the mouse brain by inhibiting the PI3K/AKT/mTOR pathway, and that Mt can effectively alleviate the oxidative stress caused by Ni, and reducing Ni induces autophagy in the mouse brain through the PI3K/AKT/mTOR pathway.

Epimutational effects of electronic cigarettes

Electronic cigarettes (e-cigarettes), since they do not require tobacco combustion, have traditionally been considered less harmful than conventional cigarettes (c-cigarettes). In recent years, however, researchers have found many toxic compounds in the aerosols of e-cigarettes, and numerous studies have shown that e-cigarettes can adversely affect the human epigenome. In this review, we provide an update on recent findings regarding epigenetic outcomes of e-cigarette aerosols. Moreover, we discussed the effects of several typical e-cigarette ingredients (nicotine, tobacco-specific nitrosamines, volatile organic compounds, carbonyl compounds, and toxic metals) on DNA methylation, histone modifications, and noncoding RNA expression. These epigenetic effects could explain some of the diseases caused by e-cigarettes. It also reminds the public that like c-cigarettes, inhaling e-cigarette aerosols could also be accompanied with potential epigenotoxicity on the human body.

Detection of DNA adduct 8-hydroxy-2'-deoxyguanosine (8-OHdG) as a toxicity bioindicator to the effects of nickel on Ni-Cr alloy prosthesis users

Previous studies have suggested that exposure to Ni from Ni-Cr alloys can affect the human body through oxidative stress. The present study discusses the effect of nickel from Ni-Cr alloy prostheses on the formation of DNA Adduct 8-Hydroxy-2′-Deoxyguanosine (8-OHdG), evaluated based on creatinine and 8-OHdG concentrations in urine, determined with LC-MS/MS, for a Ni-Cr alloy user group and a never-user control group. The mean creatinine and 8-OHdG concentrations were not significantly different between the test groups, although highest levels were observed for the in the Ni-Cr user group. It is suggested that samples with relatively high creatinine and/or 8-OHdG levels are further studied in more detail for stability of concentrations and for the effect of contributing factors.

Nuclear factor erythroid 2 - related factor 2 and its relationship with cellular response in nickel exposure: a systems biology analysis

Background

Nickel and nickel-containing compounds (NCC) are known human carcinogens. However, the precise molecular mechanisms of nickel-induced malignant transformation remain unknown. Proposed mechanisms suggest that nickel and NCC may participate in the dual activation/inactivation of enzymatic pathways involved in cell defenses against oxidative damage, where Nuclear factor-erythroid 2 related factor 2 (Nrf2) plays a central role.

Methods

For assessing the potential role of proteins involved in the Nrf2-mediated response to nickel and NCC exposure, we designed an interactome network using the STITCH search engine version 5.0 and the STRING software 10.0. The major NCC-protein interactome (NCPI) generated was analyzed using the MCODE plugin, version 1.5.1 for the detection of interaction modules or subnetworks. Main centralities of the NCPI were determined with the CentiScape 2.2 plugin of Cytoscape 3.4.0 and main biological processes associated with each cluster were assessed using the BiNGO plugin of Cytoscape 3.4.0.

Results

Water-soluble NiSO₄ and insoluble Ni₃S₂ were the most connected to proteins involved in the NCPI network. Nfr2 was detected as one of the most relevant proteins in the network, participating in several multifunctional protein complexes in clusters 1, 2, 3 and 5. Ontological analysis of cluster 3 revealed several processes related to unfolded protein response (UPR) and response to endoplasmic reticulum (ER) stress.

Conclusions

Cellular response to NCC exposure was very comparable, particularly concerning oxidative stress response, inflammation, cell cycle/proliferation, and apoptosis. In this cellular response, Nfr2 was highly centralized and participated in several multifunctional protein complexes, including several related to ER-stress. These results add evidence on the possible Ni²⁺ induced – ER stress mainly associated with insoluble NCC. In this scenario, we also show how protein degradation mediated by ubiquitination seems to play key roles in cellular responses to Ni.

Molecular Mechanisms of Nickel-Induced Carcinogenesis

BACKGROUND

The increased use of heavy metal nickel in modern industries results in increased environmental impact. Occupational and environmental exposure to nickel is closely linked to an increased risk of human lung cancer and nasal cancer.

OBJECTIVE

Unlike other heavy metal carcinogens, nickel has weak mutagenic activity. Carcinogenesis caused by nickel is intensively studied, but the precise mechanism of action is not yet known.

RESULTS

Epigenetic changes, activation of hypoxia signaling pathways, and generation of reactive oxygen species (ROS) are considered to be the major molecular mechanisms involved in nickelinduced carcinogenesis.

CONCLUSION

This review provides insights into current research on nickel-induced carcinogenesis and suggests possible effective therapeutic strategies for nickel-induced carcinogenesis.

Concise Review of Nickel Human Health Toxicology and Ecotoxicology

Nickel (Ni) metal and Ni compounds are widely used in applications like stainless steel, alloys, and batteries. Nickel is a naturally occurring element in water, soil, air, and living organisms, and is essential to microorganisms and plants. Thus, human and environmental nickel exposures are ubiquitous. Production and use of nickel and its compounds can, however, result in additional exposures to humans and the environment. Notable human health toxicity effects identified from human and/or animal studies include respiratory cancer, non-cancer toxicity effects following inhalation, dermatitis, and reproductive effects. These effects have thresholds, with indirect genotoxic and epigenetic events underlying the threshold mode of action for nickel carcinogenicity. Differences in human toxicity potencies/potentials of different nickel chemical forms are correlated with the bioavailability of the Ni2+ ion at target sites. Likewise, Ni2+ has been demonstrated to be the toxic chemical species in the environment, and models have been developed that account for the influence of abiotic factors on the bioavailability and toxicity of Ni2+ in different habitats. Emerging issues regarding the toxicity of nickel nanoforms and metal mixtures are briefly discussed. This review is unique in its covering of both human and environmental nickel toxicity data.

Molecular and epigenetic markers as promising tools to quantify the effect of occupational exposures and the risk of developing non-communicable diseases

Non-communicable diseases (NCDs) are chronic diseases that are by far the leading cause of death in the world. Many occupational hazards, together with social, economic and demographic factors, have been associated to NCDs development. Genetic susceptibility or environmental exposures alone are not usually sufficient to explain the pathogenesis of NCDs, but can be integrated in a more complex scenario that can result in pathological phenotypes. Epigenetics is a crucial component of this scenario, as its changes are related to specific exposures, therefore potentially able to display the effects of environment on the genome, filling the gap between genetic asset and environment in explaining disease development. To date, the most promising biomarkers have been assessed in occupational cohorts as well as in case/control studies and include DNA methylation, histone modifications, microRNA expression, extracellular vesicles, telomere length, and mitochondrial alterations.

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.

Nickel induces transcriptional down-regulation of DNA repair pathways in tumorigenic and non-tumorigenic lung cells

The heavy metal nickel is a known carcinogen, and occupational exposure to nickel compounds has been implicated in human lung and nasal cancers. Unlike many other environmental carcinogens, however, nickel does not directly induce DNA mutagenesis, and the mechanism of nickel-related carcinogenesis remains incompletely understood. Cellular nickel exposure leads to signaling pathway activation, transcriptional changes and epigenetic remodeling, processes also impacted by hypoxia, which itself promotes tumor growth without causing direct DNA damage. One of the mechanisms by which hypoxia contributes to tumor growth is the generation of genomic instability via down-regulation of high-fidelity DNA repair pathways. Here, we find that nickel exposure similarly leads to down-regulation of DNA repair proteins involved in homology-dependent DNA double-strand break repair (HDR) and mismatch repair (MMR) in tumorigenic and non-tumorigenic human lung cells. Functionally, nickel induces a defect in HDR capacity, as determined by plasmid-based host cell reactivation assays, persistence of ionizing radiation-induced DNA double-strand breaks and cellular hypersensitivity to ionizing radiation. Mechanistically, we find that nickel, in contrast to the metalloid arsenic, acutely induces transcriptional repression of HDR and MMR genes as part of a global transcriptional pattern similar to that seen with hypoxia. Finally, we find that exposure to low-dose nickel reduces the activity of the MLH1 promoter, but only arsenic leads to long-term MLH1 promoter silencing. Together, our data elucidate novel mechanisms of heavy metal carcinogenesis and contribute to our understanding of the influence of the microenvironment on the regulation of DNA repair pathways.

Histone 3 modifications and blood pressure in the Beijing Truck Driver Air Pollution Study

CONTEXT

Histone modifications regulate gene expression; dysregulation has been linked with cardiovascular diseases. Associations between histone modification levels and blood pressure in humans are unclear.

OBJECTIVE

We examine the relationship between global histone concentrations and various markers of blood pressure.

MATERIALS AND METHODS

Using the Beijing Truck Driver Air Pollution Study, we investigated global peripheral white blood cell histone modifications (H3K9ac, H3K9me3, H3K27me3, and H3K36me3) associations with pre- and post-work measurements of systolic (SBP) and diastolic (DBP) blood pressure, mean arterial pressure (MAP), and pulse pressure (PP) using multivariable mixed-effect models.

RESULTS

H3K9ac was negatively associated with pre-work SBP and MAP; H3K9me3 was negatively associated with pre-work SBP, DBP, and MAP; and H3K27me3 was negatively associated with pre-work SBP. Among office workers, H3K9me3 was negatively associated with pre-work SBP, DBP, and MAP. Among truck drivers, H3K9ac and H3K27me were negatively associated with pre-work SBP, and H3K27me3 was positively associated with post-work PP.

DISCUSSION AND CONCLUSION

Epigenome-wide H3K9ac, H3K9me3, and H3K27me3 were negatively associated with multiple pre-work blood pressure measures. These associations substantially changed during the day, suggesting an influence of daily activities. Blood-based histone modification biomarkers are potential candidates for studies requiring estimations of morning/pre-work blood pressure.

Blood pressure and expression of microRNAs in whole blood

Background

Blood pressure (BP) is a complex, multifactorial clinical outcome driven by genetic susceptibility, behavioral choices, and environmental factors. Many molecular mechanisms have been proposed for the pathophysiology of high BP even as its prevalence continues to grow worldwide, increasing morbidity and marking it as a major public health concern. To address this, we evaluated miRNA profiling in blood leukocytes as potential biomarkers of BP and BP-related risk factors.

Methods

The Beijing Truck Driver Air Pollution Study included 60 truck drivers and 60 office workers examined in 2008. On two days separated by 1–2 weeks, we examined three BP measures: systolic, diastolic, and mean arterial pressure measured at both pre- and post-work exams for blood NanoString nCounter miRNA profiles. We used covariate-adjusted linear mixed-effect models to examine associations between BP and increased miRNA expression in both pooled and risk factor-stratified analyses.

Results

Overall 43 miRNAs were associated with pre-work BP (FDR<0.05). In stratified analyses different but overlapping groups of miRNAs were associated with pre-work BP in truck drivers, high-BMI participants, and usual alcohol drinkers (FDR<0.05). Only four miRNAs were associated with post-work BP (FDR<0.05), in ever smokers.

Conclusion

Our results suggest that many miRNAs were significantly associated with BP in subgroups exposed to known hypertension risk factors. These findings shed light on the underlying molecular mechanisms of BP, and may assist with the development of a miRNA panel for early detection of hypertension.

Traffic-derived particulate matter exposure and histone H3 modification: A repeated measures study

BACKGROUND

Airborne particulate matter (PM) may induce epigenetic changes that potentially lead to chronic diseases. Histone modifications regulate gene expression by influencing chromatin structure that can change gene expression status. We evaluated whether traffic-derived PM exposure is associated with four types of environmentally inducible global histone H3 modifications.

METHODS

The Beijing Truck Driver Air Pollution Study included 60 truck drivers and 60 office workers examined twice, 1-2 weeks apart, for ambient PM₁₀ (both day-of and 14-day average exposures), personal PM_2.5, black carbon (BC), and elemental components (potassium, sulfur, iron, silicon, aluminum, zinc, calcium, and titanium). For both PM₁₀ measures, we obtained hourly ambient PM₁₀ data for the study period from the Beijing Municipal Environmental Bureau's 27 representatively distributed monitoring stations. We then calculated a 24h average for each examination day and a moving average of ambient PM₁₀ measured in the 14 days prior to each examination. Examinations measured global levels of H3 lysine 9 acetylation (H3K9ac), H3 lysine 9 tri-methylation (H3K9me3), H3 lysine 27 tri-methylation (H3K27me3), and H3 lysine 36 tri-methylation (H3K36me3) in blood leukocytes collected after work. We used adjusted linear mixed-effect models to examine percent changes in histone modifications per each μg/m³ increase in PM exposure.

RESULTS

In all participants each μg/m³ increase in 14-day average ambient PM₁₀ exposure was associated with lower H3K27me3 (β=-1.1%, 95% CI: -1.6, -0.6) and H3K36me3 levels (β=-0.8%, 95% CI: -1.4, -0.1). Occupation-stratified analyses showed associations between BC and both H3K9ac and H3K36me3 that were stronger in office workers (β=4.6%, 95% CI: 0.9, 8.4; and β=4.1%, 95% CI: 1.3; 7.0 respectively) than in truck drivers (β=0.1%, 95% CI: -1.3, 1.5; and β=0.9%, 95% CI: -0.9, 2.7, respectively; both p_interaction <0.05). Sex-stratified analyses showed associations between examination-day PM₁₀ and H3K9ac, and between BC and H3K9me3, were stronger in women (β=10.7%, 95% CI: 5.4, 16.2; and β=7.5%, 95% CI: 1.2, 14.2, respectively) than in men (β=1.4%, 95% CI: -0.9, 3.7; and β=0.9%, 95% CI: -0.9, 2.7, respectively; both p_interaction <0.05). We observed no associations between personal PM_2.5 or elemental components and histone modifications.

CONCLUSIONS

Our results suggest a possible role of global histone H3 modifications in effects of traffic-derived PM exposures, particularly BC exposure. Future studies should assess the roles of these modifications in human diseases and as potential mediators of air pollution-induced disease, in particular BC exposure.