Epigenetic remodeling during arsenical-induced malignant transformation

Toxicological Aspects Associated with Consumption from Electronic Nicotine Delivery System (ENDS): Focus on Heavy Metals Exposure and Cancer Risk

Tobacco smoking remains one of the leading causes of premature death worldwide. Electronic Nicotine Delivery Systems (ENDSs) are proposed as a tool for smoking cessation. In the last few years, a growing number of different types of ENDSs were launched onto the market. Despite the manufacturing differences, ENDSs can be classified as "liquid e-cigarettes" (e-cigs) equipped with an atomizer that vaporizes a liquid composed of vegetable glycerin (VG), polypropylene glycol (PG), and nicotine, with the possible addition of flavorings; otherwise, the "heated tobacco products" (HTPs) heat tobacco sticks through contact with an electronic heating metal element. The presence of some metals in the heating systems, as well as in solder joints, involves the possibility that heavy metal ions can move from these components to the liquid, or they can be adsorbed into the tobacco stick from the heating blade in the case of HTPs. Recent evidence has indicated the presence of heavy metals in the refill liquids and in the mainstream such as arsenic (As), cadmium (Cd), chromium (Cr), nickel (Ni), copper (Cu), and lead (Pb). The present review discusses the toxicological aspects associated with the exposition of heavy metals by consumption from ENDSs, focusing on metal carcinogenesis risk.

Systemic impact of heavy metals and their role in cancer development: a review

Heavy metals are well-recognised as environmental hazards due to their toxicity, environmental persistence, and bioaccumulation in living organisms. Human health is a crucial concern related to terrestrial and aquatic ecosystems poisoned by harmful heavy metals. Most heavy metals pollute the air, water, and soil, which can be fatal to humans. Humans and other species can be exposed to heavy metals through the food chain if the metals oxidise or combine with other environmental elements (such as water, soil, or air). Their entry into the food chain assures interactions with biological macromolecules in living systems, including humans, resulting in undesirable outcomes. Human poisonings have typically been caused by mercury, lead, chromium, cadmium, and arsenic. The build-up of these metals in living organisms causes various harmful consequences on different organs and tissues. The gravitas of heavy metal toxicity regarding molecular impact and carcinogenesis needs in-depth understanding despite the plethora of available data. Hence, additionally, we attempt to elaborate on the multi-level impact of five heavy metals and emphasise their role in cancer development. The rationale of this essay is thus to understand the role of five heavy metals, viz., lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and mercury (Hg), in carcinogenesis. Heavy metals interfere with various biological functions, including proliferation, differentiation, repair of damage, and apoptosis. By comparing their modes of action, we see that these metals share common mechanisms for inducing toxicity, such as reactive oxygen species (ROS) production, antioxidant defence weakening, enzyme inactivation, and oxidative stress.

Arsenic causing gallbladder cancer disease in Bihar

In recent times Gallbladder cancer (GBC) incidences increased many folds in India and are being reported from arsenic hotspots identified in Bihar. The study aims to establish association between arsenic exposure and gallbladder carcinogenesis. In the present study, n = 200 were control volunteers and n = 152 confirmed gallbladder cancer cases. The studied GBC patient's biological samples-gallbladder tissue, gallbladder stone, bile, blood and hair samples were collected for arsenic estimation. Moreover, n = 512 gallbladder cancer patients blood samples were also evaluated for the presence of arsenic to understand exposure level in the population. A significantly high arsenic concentration (p < 0.05) was detected in the blood samples with maximum concentration 389 µg/L in GBC cases in comparison to control. Similarly, in the gallbladder cancer patients, there was significantly high arsenic concentration observed in gallbladder tissue with highest concentration of 2166 µg/kg, in gallbladder stones 635 µg/kg, in bile samples 483 µg/L and in hair samples 6980 µg/kg respectively. Moreover, the n = 512 gallbladder cancer patient's blood samples study revealed very significant arsenic concentration in the population of Bihar with maximum arsenic concentration as 746 µg/L. The raised arsenic concentration in the gallbladder cancer patients' biological samples-gallbladder tissue, gallbladder stone, bile, blood, and hair samples was significantly very high in the arsenic exposed area. The study denotes that the gallbladder disease burden is very high in the arsenic exposed area of Bihar. The findings do provide a strong link between arsenic contamination and increased gallbladder carcinogenesis.

Epigenomic reprogramming in iAs-mediated carcinogenesis

Arsenic is a naturally occurring metal carcinogen found in the Earth's crust. Millions of people worldwide are chronically exposed to arsenic through drinking water and food. Exposure to inorganic arsenic has been implicated in many diseases ranging from acute toxicities to malignant transformations. Despite the well-known deleterious health effects of arsenic exposure, the molecular mechanisms in arsenic-mediated carcinogenesis are not fully understood. Since arsenic is non-mutagenic, the mechanism by which arsenic causes carcinogenesis is via alterations in epigenetic-regulated gene expression. There are two possible ways by which arsenic may modify the epigenome-indirectly through an arsenic-induced generation of reactive oxygen species which then impacts chromatin remodelers, or directly through interaction and modulation of chromatin remodelers. Whether directly or indirectly, arsenic modulates epigenetic gene regulation and our understanding of the direct effect of this modulation on chromatin structure is limited. In this chapter we will discuss the various ways by which inorganic arsenic affects the epigenome with consequences in health and disease.

Epigenetic Dysregulations in Arsenic-Induced Carcinogenesis

Arsenic is a crucial environmental metalloid whose high toxicity levels negatively impact human health. It poses significant health concerns to millions of people in developed and developing countries such as the USA, Canada, Bangladesh, India, China, and Mexico by enhancing sensitivity to various types of diseases, including cancers. However, how arsenic causes changes in gene expression that results in heinous conditions remains elusive. One of the proposed essential mechanisms that still has seen limited research with regard to causing disease upon arsenic exposure is the dysregulation of epigenetic components. In this review, we have extensively summarized current discoveries in arsenic-induced epigenetic modifications in carcinogenesis and angiogenesis. Importantly, we highlight the possible mechanisms underlying epigenetic reprogramming through arsenic exposure that cause changes in cell signaling and dysfunctions of different epigenetic elements.

Arsenic and Human Health: Genotoxicity, Epigenomic Effects, and Cancer Signaling

Arsenic is a well-known element because of its toxicity. Humans as well as plants and animals are negatively affected by its exposure. Some countries suffer from high levels of arsenic in their tap water and soils, which is considered a primary arsenic-linked risk factor for living beings. Humans generally get exposed to arsenic by contaminated drinking waters, resulting in many health problems, ranging from cancer to skin diseases. On the other hand, the FDA-certified drug arsenic trioxide provides solutions for various diseases, including several types of cancers. This issue emphasizes the importance of speciation of the metalloid elements in terms of impacts on health. When species get exposed to arsenic, it affects the cells altering their involvement. It can lead to abnormalities in inflammatory mechanisms and the immune system which contribute to the negative impacts generated on the body. The poisoning originating from arsenic gives rise to various biological signs on the body which can be useful for the diagnosis. It is important to find true biomarkers for the detection of arsenic poisoning. In view of its application in medicine and biology, studies on understanding the biological activity of arsenic have increased. In this review, we aim at summarizing the current state of knowledge of arsenic and the mechanism behind its toxicity including genotoxicity, oxidative insults, epigenomic changes, and alterations in cellular signaling.

Arsenic-induced epigenetic changes in cancer development

Arsenic is a ubiquitous metalloid whose high levels of toxicity pose major health concerns to millions of people worldwide by increasing susceptibility to various cancers and non-cancer illnesses. Since arsenic is not a mutagen, the mechanism by which it causes changes in gene expression and disease pathogenesis is not clear. One possible mechanism is through generation of reactive oxygen species. Another equally important mechanism still very much in its infancy is epigenetic dysregulation. In this review, we discuss recent discoveries underlying arsenic-induced epigenetic changes in cancer development. Importantly, we highlight the proposed mechanisms targeted by arsenic to drive oncogenic gene expression.

Metals and molecular carcinogenesis

Many metals are essential for living organisms, but at higher doses they may be toxic and carcinogenic. Metal exposure occurs mainly in occupational settings and environmental contaminations in drinking water, air pollution and foods, which can result in serious health problems such as cancer. Arsenic (As), beryllium (Be), cadmium (Cd), chromium (Cr) and nickel (Ni) are classified as Group 1 carcinogens by the International Agency for Research on Cancer. This review provides a comprehensive summary of current concepts of the molecular mechanisms of metal-induced carcinogenesis and focusing on a variety of pathways, including genotoxicity, mutagenesis, oxidative stress, epigenetic modifications such as DNA methylation, histone post-translational modification and alteration in microRNA regulation, competition with essential metal ions and cancer-related signaling pathways. This review takes a broader perspective and aims to assist in guiding future research with respect to the prevention and therapy of metal exposure in human diseases including cancer.

Dynamic Signatures of the Epigenome: Friend or Foe?

Highly dynamic epigenetic signaling is influenced mainly by (micro)environmental stimuli and genetic factors. The exact mechanisms affecting particular epigenomic patterns differ dependently on the context. In the current review, we focus on the causes and effects of the dynamic signatures of the human epigenome as evaluated with the high-throughput profiling data and single-gene approaches. We will discuss three different aspects of phenotypic outcomes occurring as a consequence of epigenetics interplaying with genotype and environment. The first issue is related to the cases of environmental impacts on epigenetic profile, and its adverse and advantageous effects related to human health and evolutionary adaptation. The next topic will present a model of the interwoven co-evolution of genetic and epigenetic patterns exemplified with transposable elements (TEs) and their epigenetic repressors Krüppel-associated box zinc finger proteins (KRAB–ZNFs). The third aspect concentrates on the mitosis-based microevolution that takes place during carcinogenesis, leading to clonal diversity and expansion of tumor cells. The whole picture of epigenome plasticity and its role in distinct biological processes is still incomplete. However, accumulating data define epigenomic dynamics as an essential co-factor driving adaptation at the cellular and inter-species levels with a benefit or disadvantage to the host.

An Overview on Arsenic Trioxide-Induced Cardiotoxicity

Arsenic trioxide (ATO) is among the first-line chemotherapeutic drugs used in oncological practice. It has shown substantial efficacy in treating patients with relapsed or refractory acute promyelocytic leukaemia. The clinical use of ATO is hampered due to cardiotoxicity and hence many patients are precluded from receiving this highly effective treatment. An alternative to this would be to use any drug that can ameliorate the cardiotoxic effects and allow exploiting the full therapeutic potential of ATO, with considerable impact on cancer therapy. Generation of reactive oxygen species is involved in a wide range of human diseases, including cancer, cardiovascular, pulmonary and neurological disorders. Hence, agents with the ability to protect against these reactive species may be therapeutically useful. The present review focuses on the beneficial as well as harmful effects of arsenic and ATO, the mechanisms underlying ATO toxicity and the possible ways that can be adopted to circumvent ATO-induced toxicity.

Autophagy in exposure to environmental chemicals

Autophagy is a catabolic pathway, which breaks down old and damaged cytoplasmic material into basic biomolecules through lysosome-mediated digestion thereby recycling cellular material. In this way, autophagy prevents the accumulation of damaged cellular components inside cells and reduces metabolic stress and toxicity. The basal level of autophagy is generally low but essential for maintaining the turnover of proteins and other molecules. The level is, however, increased in response to various stress conditions including chemical stress. This elevation in autophagy is intended to restore energy balance and improve cell survival in stress conditions. However, aberrant and/or deficient autophagy may also be involved in the aggravation of chemical-caused insults. Thus, the overall role of autophagy in chemical-induced toxicity is complex and only a limited number of environmental chemicals have been studied from this point of view. Autophagy is associated with many of the chemical-caused cytotoxic mechanisms, including mitochondrial dysfunction, DNA damage, oxidative stress, changes in the endoplasmic reticulum, impairment of lysosomal functions, and inflammation. This mini-review describes autophagy and its involvement in the responses to some common environmental exposures including airborne particulate matter, nanoparticles and tobacco smoke as well as to some common single environmental chemicals.

A review on arsenic carcinogenesis: Epidemiology, metabolism, genotoxicity and epigenetic changes

Long-term exposure to arsenic (inorganic arsenic) is a world-wide environmental health concern. Arsenic is classified as the Group 1 human carcinogen by the International Agency for Research on Cancer (IARC). Epidemiological studies have established a strong association between inorganic arsenic (iAs) exposure in drinking water and an increased incidence of cancer including bladder, liver, lung, prostate, and skin cancer. iAs also increases the risk of other diseases such as cardiovascular disease, hypertension and diabetes. The molecular mechanisms of carcinogenesis of iAs remain poorly defined, several mechanisms have been proposed, including genotoxicity, altered cell proliferation, oxidative stress, changes to the epigenome, disturbances of signal transduction pathways, cytotoxicity and regenerative proliferation. In this article, we will summarize current knowledge on the mechanisms of arsenic carcinogenesis and focus on integrating all these issues to garner a broader perspective.

DNA methylation of a non-CpG island promoter represses NQO1 expression in rat arsenic-transformed lung epithelial cells

NAD(P)H:quinone oxidoreductase 1 (NQO1), a phase II flavoenzyme that catalyzes reduction reactions to protect cells against electrophiles and oxidants, is involved in tumorigenesis. Altered methylation of the NQO1 gene has been observed and is speculated to result in aberrant NQO1 expression in rat cells undergoing chemical carcinogenesis, although this has not been proven experimentally. In this study, we first investigated the potential epigenetic mechanisms underlying the phenomenon of NQO1 differential expression in individual subclones of rat arsenic-transformed lung epithelial cells (TLECs). NQO1 expression of TLEC subclones with or without 5-aza-2'-deoxycytidine (5-Aza-CdR) treatment was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR), western blot analysis, and real-time PCR. Methylation status of the NQO1 promoter in TLEC subclones was analyzed by bisulfite sequencing. Transcriptional activity of NQO1 promoter in vitro methylated was determined by luciferase assay using a CpG-free luciferase reporter driven by the NQO1 promoter region (-435 to +229). We found that non-CpG island (non-CpGI) within the NQO1 promoter was hyper- or hypo-methylated in TLEC subclones and corresponded to low and high gene expressions, respectively. Following the treatment with 5-Aza-CdR, transcription of the NQO1 gene in the hypermethylated subclones was restored, accompanied by demethylation of the NQO1 promoter. In vitro promoter methylation almost completely silenced reporter activity in TLECs. These results indicate that DNA methylation of the non-CpGI promoter contributes to epigenetic silencing of NQO1 in rat TLECs.

Endogenous lycopene improves ethanol production under acetic acid stress in Saccharomyces cerevisiae

BACKGROUND

Acetic acid, generated from the pretreatment of lignocellulosic biomass, is a significant obstacle for lignocellulosic ethanol production. Reactive oxidative species (ROS)-mediated cell damage is one of important issues caused by acetic acid. It has been reported that decreasing ROS level can improve the acetic acid tolerance of Saccharomyces cerevisiae.

RESULTS

Lycopene is known as an antioxidant. In the study, we investigated effects of endogenous lycopene on cell growth and ethanol production of S. cerevisiae in acetic acid media. By accumulating endogenous lycopene during the aerobic fermentation of the seed stage, the intracellular ROS level of strain decreased to 1.4% of that of the control strain during ethanol fermentation. In the ethanol fermentation system containing 100 g/L glucose and 5.5 g/L acetic acid, the lag phase of strain was 24 h shorter than that of control strain. Glucose consumption rate and ethanol titer of yPS002 got to 2.08 g/L/h and 44.25 g/L, respectively, which were 2.6- and 1.3-fold of the control strain. Transcriptional changes of INO1 gene and CTT1 gene confirmed that endogenous lycopene can decrease oxidative stress and improve intracellular environment.

CONCLUSIONS

Biosynthesis of endogenous lycopene is first associated with enhancing tolerance to acetic acid in S. cerevisiae. We demonstrate that endogenous lycopene can decrease intracellular ROS level caused by acetic acid, thus increasing cell growth and ethanol production. This work innovatively   puts forward a new strategy for second generation bioethanol production during lignocellulosic fermentation.

Cyclin D1 promoter -56 and -54bp CpG un-methylation predicts invasive progression in arsenic-induced Bowen's disease

BACKGROUND

Patients with arsenic-induced Bowen's disease (As-BD) are at risk of developing invasive cancers in the skin, lung, and urinary bladder. However, a longitudinal follow-up study on the association between As-BD and invasive cancers is still lacking.

OBJECTIVES

This study aims to investigate the underlying molecular mechanisms of this malignant progression in the skin and internal organs.

METHODS

This is a biopsy-based follow-up study. We tested the DNA histograms, Cyclin D1 (CCND1) protein expression and CCND1 promoter DNA methylation in 40 pathologically confirmed specimens from As-BD patients to correlate with individual's invasive cancer occurrence in the 5-year follow-up.

RESULTS

Flow cytometric DNA histogram analysis of skin specimens showed aneuploid (n=15), G2/M arrest (n=22), and normal (n=3) DNA histograms. No patients with normal DNA histograms developed invasive cancers, whereas 13 developed invasive cancers in the aneuploid group and 2 developed invasive cancers in the G2/M arrest group. The aneuploid group showed a high risk of invasive cancer development. In all assessed aneuploid specimens, the CCND1 promoter hypomethylation was observed. Statistically, percentage of un-methylation more than 55.85% among 17 detected CpG sites showed extremely high predictive power in the occurrence of invasive arsenical cancers. Furthermore, the un-methylation at -56 and -54bp CpG sites was statistically significantly associated with invasive arsenical cancer development (p=1.29×10^-5).

CONCLUSIONS

As-BD lesions showing an aneuploid DNA histogram had a high risk of invasive cancer development. Un-methyaltion at -56 and -54bp CpG in the CCND1 promoter serves as a predictor for invasive progression in As-BD patients.

Aberrant methylation of nucleotide excision repair genes is associated with chronic arsenic poisoning

OBJECTIVE

To define whether aberrant methylation of DNA repair genes is associated with chronic arsenic poisoning.

METHODS

Hundred and two endemic arsenicosis patients and 36 healthy subjects were recruited. Methylight and bisulfite sequencing (BSP) assays were used to examine the methylation status of ERCC1, ERCC2 and XPC genes in peripheral blood lymphocytes (PBLs) and skin lesions of arsenicosis patients and NaAsO₂-treated HaCaT cells.

RESULTS

Hypermethylation of ERCC1 and ERCC2 and suppressed gene expression were found in PBLs and skin lesions of arsenicosis patients and was correlated with the level of arsenic exposure. Particularly, the expression of ERCC1 and ERCC2 was associated with the severity of skin lesions. In vitro studies revealed an induction of ERCC2 hypermethylation and decreased mRNA expression in response to NaAsO₂ treatment.

CONCLUSION

Hypermethylation of ERCC1 and ERCC2 and concomitant suppression of gene expression might be served as the epigenetic marks associated with arsenic exposure and adverse health effects.

Quantitative Mass Spectrometry Reveals Changes in Histone H2B Variants as Cells Undergo Inorganic Arsenic-Mediated Cellular Transformation

Exposure to inorganic arsenic, a ubiquitous environmental toxic metalloid, leads to carcinogenesis. However, the mechanism is unknown. Several studies have shown that inorganic arsenic exposure alters specific gene expression patterns, possibly through alterations in chromatin structure. While most studies on understanding the mechanism of chromatin-mediated gene regulation have focused on histone post-translational modifications, the role of histone variants remains largely unknown. Incorporation of histone variants alters the functional properties of chromatin. To understand the global dynamics of chromatin structure and function in arsenic-mediated carcinogenesis, analysis of the histone variants incorporated into the nucleosome and their covalent modifications is required. Here we report the first global mass spectrometric analysis of histone H2B variants as cells undergo arsenic-mediated epithelial to mesenchymal transition. We used electron capture dissociation-based top-down tandem mass spectrometry analysis validated with quantitative reverse transcription real-time polymerase chain reaction to identify changes in the expression levels of H2B variants in inorganic arsenic-mediated epithelial-mesenchymal transition. We identified changes in the expression levels of specific histone H2B variants in two cell types, which are dependent on dose and length of exposure of inorganic arsenic. In particular, we found increases in H2B variants H2B1H/1K/1C/1J/1O and H2B2E/2F, and significant decreases in H2B1N/1D/1B as cells undergo inorganic arsenic-mediated epithelial-mesenchymal transition. The analysis of these histone variants provides a first step toward an understanding of the functional significance of the diversity of histone structures, especially in inorganic arsenic-mediated gene expression and carcinogenesis.

Serum metabolomics reveals that arsenic exposure disrupted lipid and amino acid metabolism in rats: a step forward in understanding chronic arsenic toxicity

Chronic arsenic exposure through drinking water threatens public health worldwide. Although its multiorgan toxicity has been reported, the impact of chronic arsenic exposure on the metabolic network remains obscure. In this study, male Sprague Dawley rats were exposed to 0.5, 2 or 10 ppm sodium arsenite for three months. An ultra-high performance liquid chromatography/mass spectrometry based metabolomics approach was utilized to unveil the global metabolic response to chronic arsenic exposure in rats. Distinct serum metabolome profiles were found to be associated with the doses. Eighteen differential metabolites were identified, and most of them showed dose-dependent responses to arsenic exposure. Metabolic abnormalities mainly involved lipid metabolism and amino acid metabolism. The metabolic alterations were further confirmed by hepatic gene expression. Expressions of cpt2, lcat, cact, crot and mtr were significantly elevated in high dose groups. This study provides novel evidence to support the association between arsenic exposure and metabolic disruption, and it contributes to understanding the mechanism of chronic arsenic toxicity.

Arsenic Trioxide Reduces Global Histone H4 Acetylation at Lysine 16 through Direct Binding to Histone Acetyltransferase hMOF in Human Cells

Histone post-translational modification heritably regulates gene expression involved in most cellular biological processes. Experimental studies suggest that alteration of histone modifications affects gene expression by changing chromatin structure, causing various cellular responses to environmental influences. Arsenic (As), a naturally occurring element and environmental pollutant, is an established human carcinogen. Recently, increasing evidence suggests that As-mediated epigenetic mechanisms may be involved in its toxicity and carcinogenicity, but how this occurs is still unclear. Here we present evidence that suggests As-induced global histone H4K16 acetylation (H4K16ac) partly due to the direct physical interaction between As and histone acetyltransferase (HAT) hMOF (human male absent on first) protein, leading to the loss of hMOF HAT activity. Our data show that decreased global H4K16ac and increased deacetyltransferase HDAC4 expression occurred in arsenic trioxide (As₂O₃)-exposed HeLa or HEK293T cells. However, depletion of HDAC4 did not affect global H4K16ac, and it could not raise H4K16ac in cells exposed to As₂O₃, suggesting that HDAC4 might not directly be involved in histone H4K16 de-acetylation. Using As-immobilized agarose, we confirmed that As binds directly to hMOF, and that this interaction was competitively inhibited by free As₂O₃. Also, the direct interaction of As and C2CH zinc finger peptide was verified by MAIDI-TOF mass and UV absorption. In an in vitro HAT assay, As₂O₃ directly inhibited hMOF activity. hMOF over-expression not only increased resistance to As and caused less toxicity, but also effectively reversed reduced H4K16ac caused by As exposure. These data suggest a theoretical basis for elucidating the mechanism of As toxicity.

Endocrine disrupters: the new players able to affect the epigenome

Epigenetics represents the way by which the environment is able to program the genome; there are three main levels of epigenetic control on genome: DNA methylation, post-translational histone modification and microRNA expression. The term Epigenetics has been widened by NIH to include “both heritable changes in gene activity and expression but also stable, long-term alterations in the transcriptional potential of a cell that are not necessarily heritable.” These changes might be produced mostly by the early life environment and might affect health influencing the susceptibility to develop diseases, from cancer to mental disorder, during the entire life span. The most studied environmental influences acting on epigenome are diet, infections, wasting, child care, smoking and environmental pollutants, in particular endocrine disrupters (EDs). These are environmental xenobiotics able to interfere with the normal development of the male and female reproductive systems of wildlife, of experimental animals and possibly of humans, disrupting the normal reproductive functions. Data from literature indicate that EDs can act at different levels of epigenetic control, in some cases transgenerationally, in particular when the exposure to these compounds occurs during the prenatal and earliest period of life. Some of the best characterized EDs will be considered in this review. Among the EDs, vinclozolin (VZ), and methoxychlor (MXC) promote epigenetic transgenerational effects. Polychlorinated biphenils (PCBs), the most widespread environmental EDs, affect histone post-translational modifications in a dimorphic way, possibly as the result of an alteration of gene expression of the enzymes involved in histone modification, as the demethylase Jarid1b, an enzyme also involved in regulating the interaction of androgens with their receptor.

E2F1-mediated FOS induction in arsenic trioxide-induced cellular transformation: effects of global H3K9 hypoacetylation and promoter-specific hyperacetylation in vitro

BACKGROUND

Aberrant histone acetylation has been observed in carcinogenesis and cellular transformation associated with arsenic exposure; however, the molecular mechanisms and cellular outcomes of such changes are poorly understood.

OBJECTIVE

We investigated the impact of tolerated and toxic arsenic trioxide (As2O3) exposure in human embryonic kidney (HEK293T) and urothelial (UROtsa) cells to characterize the alterations in histone acetylation and gene expression as well as the implications for cellular transformation.

METHODS

Tolerated and toxic exposures of As2O3 were identified by measurement of cell death, mitochondrial function, cellular proliferation, and anchorage-independent growth. Histone extraction, the MNase sensitivity assay, and immunoblotting were used to assess global histone acetylation levels, and gene promoter-specific interactions were measured by chromatin immunoprecipitation followed by reverse-transcriptase polymerase chain reaction.

RESULTS

Tolerated and toxic dosages, respectively, were defined as 0.5 μM and 2.5 μM As2O3 in HEK293T cells and 1 μM and 5 μM As2O3 in UROtsa cells. Global hypoacetylation of H3K9 at 72 hr was observed in UROtsa cells following tolerated and toxic exposure. In both cell lines, tolerated exposure alone led to H3K9 hyperacetylation and E2F1 binding at the FOS promoter, which remained elevated after 72 hr, contrary to global H3K9 hypoacetylation. Thus, promoter-specific H3K9 acetylation is a better predictor of cellular transformation than are global histone acetylation patterns. Tolerated exposure resulted in an increased expression of the proto-oncogenes FOS and JUN in both cell lines at 72 hr.

CONCLUSION

Global H3K9 hypoacetylation and promoter-specific hyperacetylation facilitate E2F1-mediated FOS induction in As2O3-induced cellular transformation.

Involvement of epigenetics and EMT-related miRNA in arsenic-induced neoplastic transformation and their potential clinical use

Exposure to toxicants leads to cumulative molecular changes that overtime increase a subject's risk of developing urothelial carcinoma. To assess the impact of arsenic exposure at a time progressive manner, we developed and characterized a cell culture model and tested a panel of miRNAs in urine samples from arsenic-exposed subjects, urothelial carcinoma patients, and controls. To prepare an in vitro model, we chronically exposed an immortalized normal human bladder cell line (HUC1) to arsenic. Growth of the HUC1 cells was increased in a time-dependent manner after arsenic treatment and cellular morphology was changed. In a soft agar assay, colonies were observed only in arsenic-treated cells, and the number of colonies gradually increased with longer periods of treatment. Similarly, invaded cells in an invasion assay were observed only in arsenic-treated cells. Withdrawal of arsenic treatment for 2.5 months did not reverse the tumorigenic properties of arsenic-treated cells. Western blot analysis demonstrated decreased PTEN and increased AKT and mTOR in arsenic-treated HUC1 cells. Levels of miR-200a, miR-200b, and miR-200c were downregulated in arsenic-exposed HUC1 cells by quantitative RT-PCR. Furthermore, in human urine, miR-200c and miR-205 were inversely associated with arsenic exposure (P = 0.005 and 0.009, respectively). Expression of miR-205 discriminated cancer cases from controls with high sensitivity and specificity (AUC = 0.845). Our study suggests that exposure to arsenic rapidly induces a multifaceted dedifferentiation program and miR-205 has potential to be used as a marker of arsenic exposure as well as a maker of early urothelial carcinoma detection.

Epigenetics across the human lifespan

Epigenetics has the potential to explain various biological phenomena that have heretofore defied complete explication. This review describes the various types of endogenous human developmental milestones such as birth, puberty, and menopause, as well as the diverse exogenous environmental factors that influence human health, in a chronological epigenetic context. We describe the entire course of human life from periconception to death and chronologically note all of the potential internal timepoints and external factors that influence the human epigenome. Ultimately, the environment presents these various factors to the individual that influence the epigenome, and the unique epigenetic and genetic profile of each individual also modulates the specific response to these factors. During the course of human life, we are exposed to an environment that abounds with a potent and dynamic milieu capable of triggering chemical changes that activate or silence genes. There is constant interaction between the external and internal environments that is required for normal development and health maintenance as well as for influencing disease load and resistance. For example, exposure to pharmaceutical and toxic chemicals, diet, stress, exercise, and other environmental factors are capable of eliciting positive or negative epigenetic modifications with lasting effects on development, metabolism and health. These can impact the body so profoundly as to permanently alter the epigenetic profile of an individual. We also present a comprehensive new hypothesis of how these diverse environmental factors cause both direct and indirect epigenetic changes and how this knowledge can ultimately be used to improve personalized medicine.

The environmental roots of non-communicable diseases (NCDs) and the epigenetic impacts of globalization

BACKGROUND

Non-communicable diseases (NCDs) are increasing worldwide. We hypothesize that environmental factors (including social adversity, diet, lack of physical activity and pollution) can become "embedded" in the biology of humans. We also hypothesize that the "embedding" partly occurs because of epigenetic changes, i.e., durable changes in gene expression patterns. Our concern is that once such factors have a foundation in human biology, they can affect human health (including NCDs) over a long period of time and across generations.

OBJECTIVES

To analyze how worldwide changes in movements of goods, persons and lifestyles (globalization) may affect the "epigenetic landscape" of populations and through this have an impact on NCDs. We provide examples of such changes and effects by discussing the potential epigenetic impact of socio-economic status, migration, and diet, as well as the impact of environmental factors influencing trends in age at puberty.

DISCUSSION

The study of durable changes in epigenetic patterns has the potential to influence policy and practice; for example, by enabling stratification of populations into those who could particularly benefit from early interventions to prevent NCDs, or by demonstrating mechanisms through which environmental factors influence disease risk, thus providing compelling evidence for policy makers, companies and the civil society at large. The current debate on the '25 × 25 strategy', a goal of 25% reduction in relative mortality from NCDs by 2025, makes the proposed approach even more timely.

CONCLUSIONS

Epigenetic modifications related to globalization may crucially contribute to explain current and future patterns of NCDs, and thus deserve attention from environmental researchers, public health experts, policy makers, and concerned citizens.

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.

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.

The acquisition of cancer stem cell-like properties and neoplastic transformation of human keratinocytes induced by arsenite involves epigenetic silencing of let-7c via Ras/NF-κB

Exposure of humans to inorganic arsenic can cause skin cancer. The acquisition of cancer stem cell-like properties is involved in the initiation of some cancers, and there are changes in let-7 levels in some tumors. The mechanisms of action, however, remain obscure. Here, we report that there are decreased levels of let-7a, let-7b, and let-7c in human keratinocyte HaCaT cells during malignant transformation induced by a low concentration (1.0μM) of arsenite. The process by which arsenite reduces the level of let-7c apparently involves methylation, for 5-aza-2'-deoxycytidine, an inhibitor of methyltransferases, prevents arsenite-induced hypermethylation, decreases the level of let-7c, and thereby blocks arsenite-induced activation of the Ras/NF-κB signal pathway. Let-7c is an up-stream regulator of the Ras/NF-κB signal pathway and down-regulates activation of this pathway. In arsenite-transformed HaCaT cells, the acquisition of cancer stem cell-like properties is prevented by over-expression of let-7c, and over-expression of let-7c decreases the malignancy of transformed HaCaT cells. Thus, we conclude that epigenetic silencing of let-7c via Ras/NF-κB is involved in the acquisition of cancer stem cell-like properties and neoplastic transformation of HaCaT cells induced by arsenite, which contribute to the tumorigenesis of arsenite.

Genotoxic and epigenetic mechanisms in arsenic carcinogenicity

Arsenic is a human carcinogen with weak mutagenic properties that induces tumors through mechanisms not yet completely understood. People worldwide are exposed to arsenic-contaminated drinking water, and epidemiological studies showed a high percentage of lung, bladder, liver, and kidney cancer in these populations. Several mechanisms by which arsenical compounds induce tumorigenesis were proposed including genotoxic damage and chromosomal abnormalities. Over the past decade, a growing body of evidence indicated that epigenetic modifications have a role in arsenic-inducing adverse effects on human health. The main epigenetic mechanisms are DNA methylation in gene promoter regions that regulate gene expression, histone tail modifications that regulate the accessibility of transcriptional machinery to genes, and microRNA activity (noncoding RNA able to modulate mRNA translation). The "double capacity" of arsenic to induce mutations and epimutations could be the main cause of arsenic-induced carcinogenesis. The aim of this review is to better clarify the mechanisms of the initiation and/or the promotion of arsenic-induced carcinogenesis in order to understand the best way to perform an early diagnosis and a prompt prevention that is the key point for protecting arsenic-exposed population. Studies on arsenic-exposed population should be designed in order to examine more comprehensively the presence and consequences of these genetic/epigenetic alterations.

Mitochondrial oxidative stress-induced epigenetic modifications in pancreatic epithelial cells

Emerging studies have linked prooxidative carbamate compound exposures with various human pathologies including pancreatic cancer. In these studies, our aim was to examine mitochondrial oxidative stress-mediated aberrant chromatin responses in human pancreatic ductal epithelial cells. Posttranslational histone modifications, promoter DNA methylation, and micro-RNA (miRNA) expression patterns were evaluated following induction of mitochondrial oxidative stress by N-succinimidyl N-methylcarbamate exposure. In treated cells, perturbation in mitochondrial machinery led to hypermethylation of p16 and smad4 gene promoters and downregulation of respective gene products. Posttranslational histone modifications that include hypoacetylation of acetylated histone (AcH) 3 and AcH4, hypermethylation of monomethylated histone 3 at lysine 9 and trimethylated histone 4 at lysine 20 ubiquitinated histone (uH) 2A/uH2B, and increased phosphorylation of H2AX and H3 were observed in the treated cells. Altered expression of miRNAs denoted possible location of corresponding genes at oxidatively damaged fragile sites. Collectively, our results provide a direct role of mitochondrial oxidative stress-mediated epigenetic imbalance to perturbed genomic integrity in oxygen radical-induced pancreatic injury. Further, identification and characterization of molecular switches that affect these epigenomic signatures and targets thereof will be imperative to understand the complex role of redox-regulatory network in pancreatic milieu.

Toxicity of arsenite and thio-DMAV after long-term (21 days) incubation of human urothelial cells: cytotoxicity, genotoxicity and epigenetics

This study aims to further mechanistically understand toxic modes of action after chronic arsenic species exposure.

Epigenetics and epidemiology: models of study and examples

Epidemiological studies have successfully identified several environmental causes of disease, but often these studies are limited by methodological problems (e.g. lack of sensitivity and specificity in exposure assessment; confounding). Proposed approaches to improve observational studies of environmental associations are Mendelian randomization and the meet-in-the-middle (MITM) approach. The latter uses signals from the growing field of -omics as putative intermediate biomarkers in the pathogenetic process that links exposure with disease. The first part of this approach consists in the association between exposure and disease. The next step consists in the study of the relationship between (biomarkers of) exposure and intermediate -omic biomarkers of early effect; thirdly, the relation between the disease outcome and intermediate -omic biomarkers is assessed. We propose that when an association is found in all three steps it is possible that there is a casual association. One of the associations that have been investigated extensively in the recent years but is not completely understood is that between environmental endocrine disruptors and breast cancer. Here we present an example of how the "meet-in-the-middle" approach can be used to address the role of endocrine disruptors, by reviewing the relevant literature.

Coordinate H3K9 and DNA methylation silencing of ZNFs in toxicant-induced malignant transformation

Genome-wide disruption of the epigenetic code is a hallmark of malignancy that encompasses many distinct, highly interactive modifications. Delineating the aberrant epigenome produced during toxicant-mediated malignant transformation will help identify the underlying epigenetic drivers of environmental toxicant-induced carcinogenesis. Gene promoter DNA methylation and gene expression profiling of arsenite-transformed prostate epithelial cells showed a negative correlation between gene expression changes and DNA methylation changes; however, less than 10% of the genes with increased promoter methylation were downregulated. Studies described herein confirm that a majority of the DNA hypermethylation events occur at H3K27me3 marked genes that were already transcriptionally repressed. In contrast to aberrant DNA methylation targeting H3K27me3 pre-marked silent genes, we found that actively expressed C2H2 zinc finger genes (ZNFs) marked with H3K9me3 on their 3′ ends, were the favored targets of DNA methylation linked gene silencing. DNA methylation coupled, H3K9me3 mediated gene silencing of ZNF genes was widespread, occurring at individual ZNF genes on multiple chromosomes and across ZNF gene family clusters. At ZNF gene promoters, H3K9me3 and DNA hypermethylation replaced H3K4me3, resulting in a widespread downregulation of ZNF gene expression, which accounted for 8% of all the downregulated genes in the arsenical-transformed cells. In summary, these studies associate toxicant exposure with widespread silencing of ZNF genes by DNA hypermethylation-linked H3K9me3 spreading, further implicating epigenetic dysfunction as a driver of toxicant associated carcinogenesis.

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

DNA methylation plays an intricate role in the regulation of gene expression and events that compromise the integrity of the methylome may potentially contribute to disease development. DNA methylation is a reversible and regulatory modification that elicits a cascade of events leading to chromatin condensation and gene silencing. In general, normal cells are characterized by gene-specific hypomethylation and global hypermethylation, while cancer cells portray a reverse profile to this norm. The unique methylome displayed in cancer cells is induced after exposure to carcinogenic metals such as nickel, arsenic, cadmium, and chromium (VI). These metals alter the DNA methylation profile by provoking both hyper- and hypo-methylation events. The metal-stimulated deviations to the methylome are possible mechanisms for metal-induced carcinogenesis and may provide potential biomarkers for cancer detection. Development of therapies based on the cancer methylome requires further research including human studies that supply results with larger impact and higher human relevance.

Cross-contamination of a UROtsa stock with T24 cells--molecular comparison of different cell lines and stocks

BACKGROUND

UROtsa is an authentic, immortalized human urothelial cell line that is used to study the effects of metals and other toxic substances, mostly in the context of bladder cancer carcinogenesis. Unusual properties on the molecular level of a provided UROtsa cell line stock prompted us to verify its identity.

METHODS

UROtsa cell line stocks from different sources were tested on several molecular levels and compared with other cell lines. MicroRNA and mRNA expression was determined by Real-Time PCR. Chromosome numbers were checked and PCR of different regions of the large T-antigen was performed. DNA methylation of RARB, PGR, RASSF1, CDH1, FHIT, ESR1, C1QTNF6, PTGS2, SOCS3, MGMT, and LINE1 was analyzed by pyrosequencing and compared with results from the cell lines RT4, T24, HeLa, BEAS-2B, and HepG2. Finally, short tandem repeat (STR) profiling was applied.

RESULTS

All tested UROtsa cell line stocks lacked large T-antigen. STR analysis unequivocally identified our main UROtsa stock as the bladder cancer cell line T24, which was different from two authentic UROtsa stocks that served as controls. Analysis of DNA methylation patterns and RNA expression confirmed their differences. Methylation pattern and mRNA expression of the contaminating T24 cell line showed moderate changes even after long-term culture of up to 56 weeks, whereas miRNAs and chromosome numbers varied markedly.

CONCLUSIONS

It is important to check the identity of cell lines, especially those that are not distributed by major cell banks. However, for some cell lines STR profiles are not available. Therefore, new cell lines should either be submitted to cell banks or at least their STR profile determined and published as part of their initial characterization. Our results should help to improve the identification of UROtsa and other cells on different molecular levels and provide information on the use of urothelial cells for long-term experiments.

Molecular features in arsenic-induced lung tumors

Arsenic is a well-known human carcinogen, which potentially affects ~160 million people worldwide via exposure to unsafe levels in drinking water. Lungs are one of the main target organs for arsenic-related carcinogenesis. These tumors exhibit particular features, such as squamous cell-type specificity and high incidence among never smokers. Arsenic-induced malignant transformation is mainly related to the biotransformation process intended for the metabolic clearing of the carcinogen, which results in specific genetic and epigenetic alterations that ultimately affect key pathways in lung carcinogenesis. Based on this, lung tumors induced by arsenic exposure could be considered an additional subtype of lung cancer, especially in the case of never-smokers, where arsenic is a known etiological agent. In this article, we review the current knowledge on the various mechanisms of arsenic carcinogenicity and the specific roles of this metalloid in signaling pathways leading to lung cancer.

Toxicogenomic approaches for understanding molecular mechanisms of heavy metal mutagenicity and carcinogenicity

Heavy metals that are harmful to humans include arsenic, cadmium, chromium, lead, mercury, and nickel. Some metals or their related compounds may even cause cancer. However, the mechanism underlying heavy metal-induced cancer remains unclear. Increasing data show a link between heavy metal exposure and aberrant changes in both genetic and epigenetic factors via non-targeted multiple toxicogenomic technologies of the transcriptome, proteome, metabolome, and epigenome. These modifications due to heavy metal exposure might provide a better understanding of environmental disorders. Such informative changes following heavy metal exposure might also be useful for screening of biomarker-monitored exposure to environmental pollutants and/or predicting the risk of disease. We summarize advances in high-throughput toxicogenomic-based technologies and studies related to exposure to individual heavy metal and/or mixtures and propose the underlying mechanism of action and toxicant signatures. Integrative multi-level expression analysis of the toxicity of heavy metals via system toxicology-based methodologies combined with statistical and computational tools might clarify the biological pathways involved in carcinogenic processes. Although standard in vitro and in vivo endpoint testing of mutagenicity and carcinogenicity are considered a complementary approach linked to disease, we also suggest that further evaluation of prominent biomarkers reflecting effects, responses, and disease susceptibility might be diagnostic. Furthermore, we discuss challenges in toxicogenomic applications for toxicological studies of metal mixtures and epidemiological research. Taken together, this review presents toxicogenomic data that will be useful for improvement of the knowledge of carcinogenesis and the development of better strategies for health risk assessment.

Genome-wide alteration of histone H3K9 acetylation pattern in mouse offspring prenatally exposed to arsenic

Chronic exposure to arsenic in drinking water, especially in utero or perinatal exposure, can initiate neurological and cognitive dysfunction, as well as memory impairment. Several epidemiological studies have demonstrated cognitive and learning deficits in children with early exposure to low to moderate levels of arsenic, but pathogenic mechanisms or etiology for these deficits are poorly understood. Since in vivo studies show a role for histone acetylation in cognitive performance and memory formation, we examined if prenatal exposure to arsenic causes changes in the epigenomic landscape. We exposed C57Bl6/J mice to 100 μg/L arsenic in the drinking water starting 1 week before conception till birth and applied chromatin immunoprecipitation followed by high-throughput massive parallel sequencing (ChIP-seq) to evaluate H3K9 acetylation pattern in the offspring of exposed and control mice. Arsenic exposure during embryonic life caused global hypo-acetylation at H3K9 and changes in functional annotation with highly significant representation of Krüppel associated box (KRAB) transcription factors in brain samples from exposed pups. We also found that arsenic exposure of adult mice impaired spatial and episodic memory, as well as fear conditioning performance. This is the first study to demonstrate: a) genome wide changes in H3K9 acetylation pattern in an offspring prenatally exposed to arsenic, and b) a connection between moderate arsenic exposure and cognitive impairment in adult mice. The results also emphasize the applicability of Next Generation Sequencing methodology in studies aiming to reveal the role of environmental factors, other than dietary restriction, in developmental reprogramming through histone modifications during embryonic development.

Environmental toxicants, epigenetics, and cancer

Tumorigenesis, a complex and multifactorial progressive process of transformation of normal cells into malignant cells, is characterized by the accumulation of multiple cancer-specific heritable phenotypes triggered by the mutational and/or non-mutational (i.e., epigenetic) events. Accumulating evidence suggests that environmental and occupational exposures to natural substances, as well as man-made chemical and physical agents, play a causative role in human cancer. In a broad sense, carcinogenesis may be induced through either genotoxic or non-genotoxic mechanisms; however, both genotoxic and non-genotoxic carcinogens also cause prominent epigenetic changes. This review presents current evidence of the epigenetic alterations induced by various chemical carcinogens, including arsenic, 1,3-butadine, and pharmaceutical and biological agents, and highlights the potential for epigenetic changes to serve as markers for carcinogen exposure and cancer risk assessment.

Cell-type specific DNA methylation patterns define human breast cellular identity

DNA methylation plays a role in a variety of biological processes including embryonic development, imprinting, X-chromosome inactivation, and stem cell differentiation. Tissue specific differential methylation has also been well characterized. We sought to extend these studies to create a map of differential DNA methylation between different cell types derived from a single tissue. Using three pairs of isogenic human mammary epithelial and fibroblast cells, promoter region DNA methylation was characterized using MeDIP coupled to microarray analysis. Comparison of DNA methylation between these cell types revealed nearly three thousand cell-type specific differentially methylated regions (ctDMRs). MassARRAY was performed upon 87 ctDMRs to confirm and quantify differential DNA methylation. Each of the examined regions exhibited statistically significant differences ranging from 10-70%. Gene ontology analysis revealed the overrepresentation of many transcription factors involved in developmental processes. Additionally, we have shown that ctDMRs are associated with histone related epigenetic marks and are often aberrantly methylated in breast cancer. Overall, our data suggest that there are thousands of ctDMRs which consistently exhibit differential DNA methylation and may underlie cell type specificity in human breast tissue. In addition, we describe the pathways affected by these differences and provide insight into the molecular mechanisms and physiological overlap between normal cellular differentiation and breast carcinogenesis.

Aberrant overexpression of FOXM1 transcription factor plays a critical role in lung carcinogenesis induced by low doses of arsenic

Environmental or occupational exposure to low doses of arsenic induces a series of health problems including cancer. The molecular events in arsenic-induced carcinogenicity remain to be defined. In the NuLi-1 immortalized human lung epithelial cell line with p53 and pRb deficiency, exposure to low doses of arsenic trioxide for 72 h promoted cell proliferation and upregulated the gene transcription levels of FOXM1, CDC6, CDC25A, and cyclin D1, which are both critical cell cycle regulatory genes and proto-oncogenes. Continuous in vitro exposure to 1 µM arsenic trioxide for 34 wks induced malignant cell transformation, as evidenced by enhanced anchorage-independent cell growth. The expression of FOXM1, CDC6, CDC25A, and Cyclin D1 was dynamically elevated at the gene transcription and protein levels in the process of cell transformation. The carcinogenic ability of transformed cell colonies coincides with the expression levels of FOXM1 in in vitro anchorage-independent growth assays and in vivo tumor xenograft formation assays. In reverse, the knockdown of FOXM1 in lung adenocarcinoma A549 cells or arsenic-transformed NuLi-1 cells significantly decreased anchorage-independent cell growth and tumor xenograft formation. The transformed NuLi-1 cells showed genomic instability in the form of copy number variation (CNV) at chromosome 1, 5, 6, 18, and 20, but not loss of heterozygosity (LOH). These results showed for the first time that chronic exposure to low doses of arsenic trioxide promoted lung carcinogenicity, in part by aberrantly upregulating FOXM1 and its associated oncogenes, when the tumor suppressor genes p53 and pRb were inactivated.

Associations between arsenic exposure and global posttranslational histone modifications among adults in Bangladesh

BACKGROUND

Exposure to arsenic (As) is associated with an increased risk of several cancers as well as cardiovascular disease, and childhood neuro-developmental deficits. Arsenic compounds are weakly mutagenic, alter gene expression and posttranslational histone modifications (PTHMs) in vitro.

METHODS

Water and urinary As concentrations as well as global levels of histone 3 lysine 9 di-methylation and acetylation (H3K9me2 and H3K9ac), histone 3 lysine 27 tri-methylation and acetylation (H3K27me3 and H3K27ac), histone 3 lysine 18 acetylation (H3K18ac), and histone 3 lysine 4 trimethylation (H3K4me3) were measured in peripheral blood mononuclear cells (PBMC) from a subset of participants (N = 40) of a folate clinical trial in Bangladesh (FACT study).

RESULTS

Total urinary As (uAs) was positively correlated with H3K9me2 (r = 0.36, P = 0.02) and inversely with H3K9ac (r = -0.47, P = 0.002). The associations between As and other PTHMs differed in a gender-dependent manner. Water As (wAs) was positively correlated with H3K4me3 (r = 0.45, P = 0.05) and H3K27me3 (r = 0.50, P = 0.03) among females and negatively correlated among males (H3K4me3: r = -0.44, P = 0.05; H3K27me3: r = -0.34, P = 0.14). Conversely, wAs was inversely associated with H3K27ac among females (r = -0.44, P = 0.05) and positively associated among males (r = 0.29, P = 0.21). A similar pattern was observed for H3K18ac (females: r = -0.22, P = 0.36; males: r = 0.27, P = 0.24).

CONCLUSION

Exposure to As is associated with alterations of global PTHMs; gender-specific patterns of association were observed between As exposure and several histone marks.

IMPACT

These findings contribute to the growing body of evidence linking As exposure to epigenetic dysregulation, which may play a role in the pathogenesis of As toxicity.