Arsenic: Extension of its Endocrine Disruption Potential to Interference with Estrogen Receptor-Mediated Signaling
Division of Toxicological Sciences, Department of Environmental Health Sciences, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205
1 For correspondence via e-mail: wwatson{at}jhsph.edu.
Received April 30, 2007; accepted May 2, 2007
Human exposure to arsenic (As) has become an important public health concern. Exposures most commonly occur through consumption of drinking water where the current maximum contaminant level of As, set by the U.S. Environmental Health Administration and the World Health Organization, is 10 µg/l. However, contamination from natural sources in various areas of the United States such as New England and the Southwest, and regions of the world, such as Bangladesh and Taiwan can reach hundreds of micrograms/liter in wells. Exposures can also occur in occupational settings where arsenic is used to manufacture pesticides and wood preservatives, and superfund sites where industrial waste was disposed. An example of the issue and concern is represented by this recent headline which appeared in the Baltimore Sun on Friday, 20 April 2007. "ARSENIC FORCES CLOSING OF PARK—Tests show high levels in soil; city locks gates of Swann Park." Swann Park is located next to a site where arsenic was used in the manufacture of pesticides.
Exposure to high levels of As has been associated with various human diseases including cardiovascular and diabetes, and cancers of the skin, lung, bladder, kidney, and liver (see reviews by Huang et al., 2004
; Rossman, 2003; Tapio and Grosche, 2006). Recent systematic reviews of the epidemiologic evidence for associations of As exposure with cardiovascular disease (Navas-Acien et al., 2005) and diabetes (Navas-Acien et al., 2006) detected increased risks associated with the highest As exposure categories in Taiwan, but were inconclusive in other populations. While the Taiwan studies suffered from several methodological limitations that made exposure estimates uncertain, the observations from these studies indicate the importance of conducting additional, well-designed investigations. In New Hampshire, where drinking water levels are commonly elevated, an association was observed between the highest category of As exposure, as determined by As levels in toenail clippings, and bladder cancer among smokers, but not among never smokers (Karagas et al., 2004). The authors concluded that "ingestion of low to moderate arsenic levels may affect bladder cancer incidence, and that cigarette smoking may act as a co-carcinogen" (Karagas et al., 2004). This latter observation is particularly interesting in light of the findings that As exposure has been associated with decreased expression of the DNA excision repair gene ERCC1 (excision repair cross-complement 1) and decreased DNA repair capacity in human lymphocytes from As-exposed individuals (Andrew et al., 2006), representing a possible, but not necessarily exclusive, mechanism by which As exposure could exert cocarcinogenic effects.
The metabolism of As almost certainly contributes to the adverse health effects seen in populations exposed to inorganic arsenic. Inorganic arsenic (iAs) exists in two valence states: iAs(III) and iAs(V). In humans, iAs(III) is oxidatively methylated to monomethyl As(V), reduced to monomethyl As(III), methylated again to dimethyl As(V), and then reduced to dimethyl As(III). In general, trivalent As species are more toxic than pentavalent species, and organic forms are more toxic than inorganic forms. The enzyme responsible for both methylation steps is arsenic(III)methyltransferase (AS3MT) (Li et al., 2005). Several enzymes have been shown to reduce pentavalent As species to trivalent As, but recent evidence suggests that AS3MT itself may be capable of catalyzing the reduction, provided there is a source of reducing equivalents such as glutathione or thioredoxin. Differences in As metabolism may contribute to differences in individual susceptibility to disease development. In addition, several polymorphisms have been identified in AS3MT and several classes of glutathione S-transferases that influence the profile of urinary As metabolites and contribute to differences in susceptibility to adverse health effects associated with As exposure (Engstrom et al., 2007; McCarty et al., 2007). The metabolism of As and its contribution to adverse effects is an area of active investigation, and experimental studies may contribute to differences observed in the effects of As in different cells lines.
While As is a human carcinogen, it is difficult to categorize it as an initiator, promoter, or cocarcinogen. One of the biggest obstacles to defining the mechanism of As carcinogenesis has been the lack of good animal models (Huang et al., 2004; Rossman, 2003). Recently, Waalkes et al. (2003) developed a model of transplacental As exposure that yields tumors of the liver, lung, ovary, and adrenal in adult C3H mice. Microarray analyses of fetal liver tissue of mice exposed to As transplacentally from gestational days 8–18 revealed widespread changes in the expression of genes involved in estrogen metabolism and estrogen responsiveness, suggesting a role for abnormal estrogen signaling in this model (Liu et al., 2007). Comprehensive studies using this model may provide important new insight into the mechanisms of As carcinogenesis and susceptibility at critical life stages. Although As does not cause point mutations, it has been reported to have genotoxic effects as represented by detection of deletions, chromosome aberrations, aneuploidy, micronuclei, and other types genetic damage in vivo and in cultured cells (Huang et al., 2004; Rossman, 2003). Elevated levels of reactive oxygen species (ROS) and oxidative modifications of DNA have been observed in arsenic-exposed cells (Barchowsky et al., 1999; Gebel, 2002). The source of the ROS is unclear but the repeated oxidation and reduction of As during its metabolism may contribute.
As causes numerous other effects including alterations of stress-response signaling pathways at the transcriptional level, perhaps mediated through increased ROS levels, and effects on signaling through protein kinase pathways, depending on concentration and cell system being used (Barchowsky et al., 1999; Huang et al., 2004; Tapio and Grosche, 2006). However, some findings with As have been conflicting. In one study in MCF-7 cells, As inhibited E2-induced cell proliferation at low concentrations (0.25–1µM) and stimulated apoptosis at higher concentration (2µM) (Chow et al., 2004a,b). In contrast, in another study 1µM As alone stimulated MCF-7 cell estrogen receptor (ER)–mediated (i.e., effect inhibited by the specific antiestrogen ICI 182,780) cell proliferation and gene expression (Stoica et al., 2000). In yet another study in porcine aortic endothelial cells, low concentrations (1µM) of As stimulated and higher concentrations (10µM) inhibited cell proliferation (Barchowsky et al., 1999). Another area where there are conflicting results pertains to whether As exhibits competitive binding to the ER. In one study using MCF-7 cells extracts, As was demonstrated to inhibit E2 binding to the ER
with a Ki of 5nM (Stoica et al., 2000), whereas in another study using recombinant ER, As did not show competitive binding (Chow et al., 2004b). Clearly, additional work needs to be done to clarify and definitively determine the effects of As on ER-mediated processes and the mechanisms.
Hamilton and coworkers have conducted a series of investigations on the mechanisms by which environmentally relevant concentrations As may cause adverse health effects and the results suggest that one mechanism is through endocrine disruption (Kaltreider et al., 2001). In a series of investigations in cultured mammalian cells they have demonstrated that As exposure alters glucocorticoid induction of gene expression mediated through the glucocorticoid receptor (GR). The alteration is characterized by what appears to be a biphasic dose–response with enhancement of GR-mediated glucocorticoid hormone induction of both endogenous and reporter gene expression at low arsenic concentrations and inhibition at higher concentrations (Bodwell et al., 2004). Similar biphasic dose–responses were reported for the effects of As on the receptors for androgens, mineralocorticoids, and progestins (Bodwell et al., 2006). However, the extent of the low concentration stimulation observed was dependent on the cellular level of GR introduced into the cell lines by transfection. That is, a reduction in the stimulatory response at low As concentrations was associated with increasing amounts of GR (Bodwell et al., 2004). Another interesting observation was that the GR-mediated inhibition of AP1 and nuclear factor kappaB (NF-
B)–mediated gene expression was not affected by As (Bodwell et al., 2004) demonstrating a degree of selectivity. Mechanism studies conducted by these investigators suggest that As disrupts the assembly of an active transcriptional complex as opposed to direct effects on ligand binding to the GR (Bodwell et al., 2004).
In this issue, Davey et al. (2007) have extended their investigations on the potential for As to disrupt the effects of estradiol (E2). They observed a concentration-dependent inhibition of E2-induced gene expression mediated by the ER both in vivo in the chick embryo and in the human breast tumor-derived MCF-7 cell line, where E2-induced expression of a transfected ERE (estrogen response element)-luc reporter gene and of the endogenous E2 responsive gene, GREB1, were used as end points. Inhibition was detected at 0.25µM As with the EC50 of approximately 2.5µM for E2-induced ERE-luc expression and 5.0µM for GREB1 expression. Upon simultaneous treatment with E2 + As, significant inhibition of ERE-luc expression was first observed after 6 h and increased progressively thereafter. Furthermore, As was observed to inhibit E2-induced ERE-luc expression whether added 4 h prior to or up to 4 h after E2 treatment, suggesting to the authors that As caused an inhibition of ongoing transcription. Low concentrations of As were not observed to increase ER-mediated gene expression, which is in contrast to what was observed for GR-mediated gene expression, as mentioned above. However, enhancement of GR-mediated gene expression by low As concentrations was dependent on the level of human GR expression, and perhaps the ER expression level in MCF-7 cells is high and thus masks a potential stimulatory effect. It now becomes important to investigate the detailed mechanism(s) of As effects on ER-mediated gene expression.
It is now well established that cellular responses to estrogens is mediated through multiple signaling pathways. ER and ERß not only function in the nucleus, but are also imported into mitochondria and become associated with the plasma membrane. The nuclear response to E2 is mediated through direct interaction of ERs with ERE as well as indirectly through interaction of ERs with other transcription factors (e.g., Sp1, AP-1, NF-B). In both cases these interactions are followed by associations with an array of coregulatory and transcriptional machinery proteins (Ascenzi et al., 2006). Import into mitochondria is followed by increased levels of mitochondrial DNA-encoded mRNA levels, presumably mediated through ER binding to ERE-like elements in mitochondrial DNA (Chen et al., 2004b). The association of the ERs with the plasma membrane leads within minutes of E2 treatment, to increases in various protein kinase pathways and levels of second messengers (Chen et al., 2004a; Li et al., 2006; Marino et al., 2006; Pedram et al., 2006; Song et al., 2005). Mitogen-activated protein kinases when activated can lead to the phosphorylation of numerous substrates (Shah and Catt, 2006), demonstrating the vast network of signaling pathways potentially affected by estrogens. The complexity of and specific roles for these nonnuclear ER signaling pathways are being elucidated (Yager and Chen, 2007).
The mechanism(s) by which As affects ER-mediated gene expression remains to be determined, but the data on the constellation of effects As exerts provide several hints (Fig. 1). A critical finding in the study by Davey et al. (2007) is that As appeared to be inhibiting ongoing transcription. That is, reporter gene expression measured 24 h after E2 treatment was similar whether As was added 4 h prior or 4 h after E2. Evidence reviewed in Ascenzi et al. (2006) suggests that ER may cycle on and off the promoter, a process that may "... allow for continuous sampling of E2 levels." Such a process would provide opportunity for As-induced modifications of the ER and/or coregulatory proteins by ROS, changes in phosphorylation, or even direct binding by As. Future studies will have to explore these possibilities in a well-defined experimental system.
|
REFERENCES
Andrew AS, Burgess JL, Meza MM, Demidenko E, Waugh MG, Hamilton JW, Karagas MR. Arsenic exposure is associated with decreased DNA repair in vitro and in individuals exposed to drinking water arsenic. Environ. Health Perspect. (2006) 114:1193–1198.[CrossRef][ISI][Medline]
Ascenzi P, Bocedi A, Marino M. Structure-function relationship of estrogen receptor alpha and beta: Impact on human health. Mol. Aspects Med. (2006) 27:299–402.[CrossRef][Medline]
Barchowsky A, Roussel RR, Klei LR, James PE, Ganju N, Smith KR, Dudek EJ. Low levels of arsenic trioxide stimulate proliferative signals in primary vascular cells without activating stress effector pathways. Toxicol. Appl. Pharmacol. (1999) 159:65–75.[CrossRef][ISI][Medline]
Bodwell JE, Gosse JA, Nomikos AP, Hamilton JW. Arsenic disruption of steroid receptor gene activation: Complex dose-response effects are shared by several steroid receptors. Chem. Res. Toxicol. (2006) 19:1619–1629.[CrossRef][ISI][Medline]
Bodwell JE, Kingsley LA, Hamilton JW. Arsenic at very low concentrations alters glucocorticoid receptor (GR)-mediated gene activation but not GR-mediated gene repression: Complex dose-response effects are closely correlated with levels of activated GR and require a functional GR DNA binding domain. Chem. Res. Toxicol. (2004) 17:1064–1076.[CrossRef][ISI][Medline]
Chen J, Lipska BK, Halim N, Ma QD, Matsumoto M, Melhem S, Kolachana BS, Hyde TM, Herman MM, Apud J, et al. Functional analysis of genetic variation in catechol-O-methyltransferase (COMT): Effects on mRNA, protein, and enzyme activity in postmortem human brain. Am. J. Hum. Genet. (2004a) 75:807–821.[CrossRef][ISI][Medline]
Chen JQ, Eshete M, Alworth WL, Yager JD. Binding of MCF-7 cell mitochondrial proteins and recombinant human estrogen receptors alpha and beta to human mitochondrial DNA estrogen response elements. J. Cell. Biochem. (2004b) 93:358–373.[CrossRef][ISI][Medline]
Chow SK, Chan JY, Fung KP. Inhibition of cell proliferation and the action mechanisms of arsenic trioxide (As2O3) on human breast cancer cells. J. Cell. Biochem. (2004a) 93:173–187.[CrossRef][ISI][Medline]
Chow SK, Chan JY, Fung KP. Suppression of cell proliferation and regulation of estrogen receptor alpha signaling pathway by arsenic trioxide on human breast cancer MCF-7 cells. J. Endocrinol. (2004b) 182:325–337.[Abstract]
Davey JC, Bodwell JE, Gosse JA, Hamilton JW. Arsenic as an endocrine disruptor: Effects of Arsenic on estrogen receptor-mediated gene expression in vivo and in cell culture. Toxicol. Sci. (2007).
Engstrom KS, Broberg K, Concha G, Nermell B, Warholm M, Vahter M. Genetic polymorphisms influencing arsenic metabolism: Evidence from Argentina. Environ. Health Perspect. (2007) 115:599–605.[ISI][Medline]
Gebel TW. Arsenic methylation is a process of detoxification through accelerated excretion. Int. J. Hyg. Environ. Health (2002) 205:505–508.[CrossRef][ISI][Medline]
Huang C, Ke Q, Costa M, Shi X. Molecular mechanisms of arsenic carcinogenesis. Mol. Cell. Biochem. (2004) 255:57–66.[CrossRef][ISI][Medline]
Kaltreider RC, Davis AM, Lariviere JP, Hamilton JW. Arsenic alters the function of the glucocorticoid receptor as a transcription factor. Environ. Health Perspect. (2001) 109:245–251.[ISI][Medline]
Karagas MR, Tosteson TD, Morris JS, Demidenko E, Mott LA, Heaney J, Schned A. Incidence of transitional cell carcinoma of the bladder and arsenic exposure in New Hampshire. Cancer Causes Control (2004) 15:465–472.[CrossRef][ISI][Medline]
Li J, Waters SB, Drobna Z, Devesa V, Styblo M, Thomas DJ. Arsenic (+3 oxidation state) methyltransferase and the inorganic arsenic methylation phenotype. Toxicol. Appl. Pharmacol. (2005) 204:164–169.[CrossRef][ISI][Medline]
Li X, Zhang S, Safe S. Activation of kinase pathways in MCF-7 cells by 17beta-estradiol and structurally diverse estrogenic compounds. J. Steroid Biochem. Mol. Biol. (2006) 98:122–132.[CrossRef][ISI][Medline]
Liu J, Xie Y, Cooper R, Ducharme DM, Tennant R, Diwan BA, Waalkes MP. Transplacental exposure to inorganic arsenic at a hepatocarcinogenic dose induces fetal gene expression changes in mice indicative of aberrant estrogen signaling and disrupted steroid metabolism. Toxicol. Appl. Pharmacol (2007) 220:284–291.[CrossRef][Medline]
Marino M, Ascenzi P, Acconcia F. S-palmitoylation modulates estrogen receptor alpha localization and functions. Steroids (2006) 71:298–303.[CrossRef][ISI][Medline]
McCarty KM, Chen YC, Quamruzzaman Q, Rahman M, Mahiuddin G, Hsueh YM, Su L, Smith T, Ryan L, Christiani DC. Arsenic methylation, GSTT1, GSTM1, GSTP1 polymorphisms, and skin lesions. Environ. Health Perspect. (2007) 115:341–345.[ISI][Medline]
Navas-Acien A, Sharrett AR, Silbergeld EK, Schwartz BS, Nachman KE, Burke TA, Guallar E. Arsenic exposure and cardiovascular disease: A systematic review of the epidemiologic evidence. Am. J. Epidemiol. (2005) 162:1037–1049.
Navas-Acien A, Silbergeld EK, Streeter RA, Clark JM, Burke TA, Guallar E. Arsenic exposure and type 2 diabetes: A systematic review of the experimental and epidemiological evidence. Environ. Health Perspect. (2006) 114:641–648.[ISI][Medline]
Pedram A, Razandi M, Wallace DC, Levin ER. Functional estrogen receptors in the mitochondria of breast cancer cells. Mol. Biol. Cell (2006) 17:2125–2137.
Rossman TG. Mechanism of arsenic carcinogenesis: An integrated approach. Mutat. Res. (2003) 533:37–65.[Medline]
Shah BH, Catt KJ. Protein phosphatase 5 as a negative key regulator of Raf-1 activation. Trends Endocrinol. Metab. (2006) 17:382–384.[CrossRef][ISI][Medline]
Song RX, Zhang Z, Mor G, Santen RJ. Down-regulation of Bcl-2 enhances estrogen apoptotic action in long-term estradiol-depleted ER(+) breast cancer cells. Apoptosis (2005) 10:667–678.[CrossRef][ISI][Medline]
Stoica A, Pentecost E, Martin MB. Effects of arsenite on estrogen receptor-alpha expression and activity in MCF-7 breast cancer cells. Endocrinology (2000) 141:3595–3602.
Tapio S, Grosche B. Arsenic in the aetiology of cancer. Mutat. Res. (2006) 612:215–246.[CrossRef][ISI][Medline]
Waalkes MP, Ward JM, Liu J, Diwan BA. Transplacental carcinogenicity of inorganic arsenic in the drinking water: Induction of hepatic, ovarian, pulmonary, and adrenal tumors in mice. Toxicol. Appl. Pharmacol. (2003) 186:7–17.[CrossRef][ISI][Medline]
Yager JD, Chen JQ. Mitochondrial estrogen receptors - new insights into specific functions. Trends Endocrinol. Metab (2007) 18:89–91.[CrossRef][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||