Effects of Chronic Exposure to Arsenic and Estrogen on Epigenetic Regulatory Genes Expression and Epigenetic Code in Human Prostate Epithelial Cells

Singh KP (2012) Effects of Chronic Exposure to Arsenic and Estrogen on Epigenetic Regulatory Genes Expression and Epigenetic Code in Human Prostate Epithelial Cells. PLoS ONE 7(8): e43880. doi:10.1371/journal.pone.0043880 Effects of Chronic Exposure to Arsenic and Estrogen on Epigenetic Regulatory Genes Expression and Epigenetic Code in Human Prostate Epithelial Cells Justin N. Treas 0 Tulika Tyagi 0 Kamaleshwar P. Singh 0 Gokul M. Das, Roswell Park Cancer Institute, United States of America 0 Department of Environmental Toxicology, The Institute of Environmental and Human Health (TIEHH), Texas Tech University , Lubbock, Texas , United States of America Chronic exposures to arsenic and estrogen are known risk factors for prostate cancer. Though the evidence suggests that exposure to arsenic or estrogens can disrupt normal DNA methylation patterns and histone modifications, the mechanisms by which these chemicals induce epigenetic changes are not fully understood. Moreover, the epigenetic effects of coexposure to these two chemicals are not known. Therefore, the objective of this study was to evaluate the effects of chronic exposure to arsenic and estrogen, both alone and in combination, on the expression of epigenetic regulatory genes, their consequences on DNA methylation, and histone modifications. Human prostate epithelial cells, RWPE-1, chronically exposed to arsenic and estrogen alone and in combination were used for analysis of epigenetic regulatory genes expression, global DNA methylation changes, and histone modifications at protein level. The result of this study revealed that exposure to arsenic, estrogen, and their combination alters the expression of epigenetic regulatory genes and changes global DNA methylation and histone modification patterns in RWPE-1 cells. These changes were significantly greater in arsenic and estrogen combination treated group than individually treated group. The findings of this study will help explain the epigenetic mechanism of arsenic- and/or estrogen-induced prostate carcinogenesis. - Competing Interests: Co-author Kamaleshwar P. Singh is a PLoS ONE Editorial Board member. This does not alter the authors adherence to all the PLoS ONE policies on sharing data and materials. Prostate cancer is the second leading cause of cancer death in American men and both endogenous and exogenous factors are involved in prostate carcinogenesis [1]. Hormonal imbalance occurring with old age is associated with the high incidence of prostate cancer (1 in 100) in men over the age of 65 [2]. Prolonged exposure to elevated levels of natural estrogen, 17b-estradiol, or xenoestrogen plays significant role in development and growth of prostate and induction of prostate cancer [3,4]. For example, fetal exposure to the xenoestrogens ethinylestradiol and bisphenol A increase the size of prostate at adult age in mice [5]. Neonatal exposure to 17b-estradiol (E2), or synthetic estrogen, diethylstilbestrol, causes increased incidence of prostate intraepithelial hyperplasia (PIN), a preneoplastic lesion in the prostate of mice [6]. Moreover, exposure to higher levels of estrogen during early developmental period predisposes to prostate cancer development later in life [6]. Similarly, epidemiological and experimental studies suggest an association between prostate cancer and chronic exposure to inorganic arsenic [7]. Humans are exposed to arsenic primarily through drinking water and inhalation of contaminated dust from coal burning. Increased prostate cancer incidence and mortality in certain US population that were exposed to higher levels of arsenic through their drinking water, further indicates a strong association between arsenic exposure and prostate cancer [8]. Fetal and/or neonatal exposure to arsenic causes cancers, including that of the prostate at adulthood in mice [9]. Effects of arsenic on the expression of several genes and their role in neoplastic transformation of cells are well documented [10,11]. However, the mechanism by which arsenic and/or estrogen exposure contributes to the development of prostate cancer is not well understood. Moreover, the effects of the combined exposures to these two chemicals on prostate epithelial cells are not known. Chromatin remodeling by epigenetic reprogramming controls the regulation of gene expression and has important implications in development of human cancers [12,13]. For example, DNA methylation can contribute to carcinogenesis in several ways including loss of imprinting, generation of chromosomal instability, re-activation of transposons, and activation of normally methylated oncogenes [14]. Post-translational modifications of histones and DNA methylation changes are two important epigenetic mechanisms that control active or passive chromatin structure and gene expression [15]. Alterations in DNA methlyation and histone modification patterns have been extensively reported in cancerous tissue [12]. Therefore, evaluation of the effects of arsenic and estrogen on expression of these genes could be helpful in understanding the mechanism of arsenic and/or estrogen induced prostate cancer development. DNA methylation plays an important role in maintaining normal expression of genes and genomic stability by controlling expression of tumor suppressor genes and repression of repetitive sequences that may otherwise cause genomic instability [16]. DNA methylation is regulated by DNA methyltransferases (DNMTs) and altered expression of DNMTs has been reported in human tumor tissue [17]. Normal DNA methylation patterns are maintained by DNMT1 [13], whereas DNMT3a and 3b nondiscriminately methylate unmethylated cytosine or hemimethylated cytosine in the genome [18]. Additionally, methyl binding proteins, MBD1-4 and MeCP2, also play an important role in epigenetic programming by recognizing and binding to CpG islands [13,19]. Post-translational modifications, such as, acetylation and methylation of histones are another epigenetic mechanism for regulation of gene expression [20]. Deacetylation of the histone at N-terminal tail by histone deacetylases (HDACs) causes compact chromatin structure leading to transcriptional repression of genes [20], whereas its acetylation by histone acetyltransferases (HATs) results in a loose chromatin structure that facilitates increased gene transcription [21]. Similarly, histone methylation mediated by histone methyltransferases (HMTs) also plays an important role in gene activation and suppression depending upon target amino acids for methylation [12,22]. Accumulating evidence strongly indicates that prostate cancer is driven by accumulation of genetic and epigenetic aberrations [23]. Apart from global hypomethylation, specific genes like GSTP1, APC, MDR1, 14-3-3s, GPX3 have been shown to be inactivated by hypermethylation in prostate cancer [24]. Another study has shown that the frequent loss of E-cadherin in prostate cancer cell lines results from hypermethylation [25] (...truncated)