Prenatal arsenic exposure alters the programming of the glucocorticoid signaling system during embryonic development
Introduction
Early developmental and adult exposure to arsenic is associated with a multitude of health problems, including cognitive (Rodriguez-Barranco et al., 2013, Rosado et al., 2007, von Ehrenstein et al., 2007, Wasserman et al., 2004), cardiovascular (McClintock et al., 2014, Moon et al., 2012, Wu et al., 2014), metabolic (Chen et al., 2009, Joshi and Shrestha, 2010, Lu et al., 2014) and metastatic (Ferreccio et al., 2013, Mostafa and Cherry, 2013, Wong et al., 1998, Xie et al., 2014) disorders that are manifested across the lifespan. The developmental origins of health and disease (DOHaD) theory (Gluckman et al., 2007, McMullen and Mostyn, 2009, Wadhwa et al., 2009) proposes that susceptibilities to chronic diseases are, in part, determined by physiologic changes initiated in utero in response to an altered prenatal environment. The theory posits that an adverse intra-uterine environment programs the development of the fetus to accommodate for this toxic environment by altering gene expressions. These adaptations are likely through epigenetic modifications. One system, the glucocorticoid signaling pathway has been demonstrated to be programmed in response to prenatal environments (Bolten et al., 2013, Conradt et al., 2013, Khulan and Drake, 2012, Lukaszewski et al., 2013, Reynolds, 2013) and these programming changes have been observed into adulthood (Goldstein et al., 2014, Khalife et al., 2013), including the transmission of the altered programming across generations (Luo et al., 2014). Further, there is support for the suggestion that altered glucocorticoid receptor (GR) programming could impact adult health (Brunton and Russell, 2011, Harris and Seckl, 2011, Merlot et al., 2008, Sarkar et al., 2008).
While there are many systems affected by arsenic, few have been implicated to play a role in all the various health problems associated with exposure. The GR signaling system, has been linked with cognitive deficits (Rodriguez et al., 2011), cardiovascular (Santos and Joles, 2012), metabolic (Sarr et al., 2012, Spencer, 2012), immune (Bellavance and Rivest, 2014) and metastatic diseases (Kitchin and Wallace, 2008, Schmitz et al., 2009). Several studies have demonstrated that arsenic produces specific disruptions in glucocorticoid transcriptional activity and steroid receptor function (Ahir et al., 2013, Barr et al., 2009, Bodwell et al., 2004, Davey et al., 2007, Gosse et al., 2014, Hamilton et al., 1998, Kaltreider et al., 2001, Shaw et al., 2007). Many of these studies have utilized cell lines or cultured cells, which provide a great deal of mechanistic information regarding the impact of toxins on specific cellular regulators but precludes the ability to assess the impact of arsenic on developmental programming including the interaction between maternal and fetal physiology.
Our own work using an in vivo mouse model exposed prenatally to 50 ppb arsenic in drinking water has found alterations in the levels of glucocorticoid signaling (Goggin et al., 2012, Martinez-Finley et al., 2009, Martinez et al., 2008), in the hippocampus from adolescent offspring. While it is clear that arsenic, both in vivo and in vitro, produces significant changes in the glucocorticoid system, the impact of arsenic on the glucocorticoid system in both the developing fetus and the placenta have not been explored. Fetal levels of glucocorticoid are regulated by both the fetal and the placental tissues through the developmentally regulated isozymes, 11β-hydroxysteroid dehydrogenases (11β-HSD) 1 and 2, which interconvert active and inactive cortisol (corticosterone). In the mouse, placental expression of 11β-HSD2 is elevated until about embryonic day 13 (E13), when it serves to inactivate maternal corticosterone and protects the developing fetus from the high glucocorticoid activation. Expression of 11β-HSD1 begins around E14 and peaks at E18 (Brown et al., 1996, Speirs et al., 2004). 11β-HSD1 is a reductase, which activates corticosterone and regenerates the glucocorticoid increasing fetal corticosterone levels. The developmental expression and levels of these isozymes are seen as being a critical regulation point for hypothalamic–pituitary–adrenal (HPA) axis negative feedback (Brunton and Russell, 2011, Chapman et al., 2013, Reichardt and Schutz, 1996). Maternal exposure to arsenic may alter the timing of 11β-HSD isozyme expression levels in an attempt to adapt to the influence of arsenic on GR signaling. Additionally, arsenic readily crosses the placenta (Caumette et al., 2007, Fei et al., 2013, He et al., 2007, Vahter, 2009) and might directly alter expression and levels of the GR signaling complex within the developing fetal brain. Little is known about the impact of arsenic in utero on the developing fetal brain and placenta.
In the present study, we evaluated the effect of 50 ppb (50 μg/L) arsenic on the glucocorticoid signaling system in both the fetal brain and placenta at two development time-points (E14 and E18) to identify critical shifts in the expression of signaling components which might impact the GR programming and later adult stress responding. The embryonic period from E14 to E18 is critical for the expression shift between 11β-HSD2 and 11β-HSD1 and for development of the negative feedback system for the centrally regulated HPA axis. If prenatal arsenic alters the level and/or the timing of the expression of these proteins which regulate the glucocorticoid response, then it may alter the central regulation of HPA axis programming and affect stress responding throughout life.
Section snippets
Prenatal arsenic exposure paradigm
All procedures were conducted in accordance with protocols approved by the Institutional Animal Care and Use Committee at the University of New Mexico. Animals were maintained in a 22 °C vivarium on a reverse light/dark cycle with lights on at 2000 and ad libitum access to water and food. Exposure to arsenic was performed as previously described during all three trimesters of development (Martinez et al., 2008, Tyler et al., 2014). Briefly, singly housed female C57BL/6J mice (Jackson
Levels of arsenic species (As5, MMA and DMA) were elevated in the brains from arsenic exposed fetuses relative to controls
Both the reduced form of inorganic arsenic, trivalent (As (III)) and the oxidized pentavalent (As (V)) form, accumulate in tissues. Metabolism of arsenic by oxidative methylation leads to the formation of monomethylarsenicals (MMA III and MMA V) and dimethylated arsenicals (DMA III and DMA V). The levels of the trioxide form, arsenite, were not significantly different between the two exposure groups (Table 2).
Prenatal arsenic did not alter the number of fetuses, crown to rump length or placental weight at the specific gestational time points tested
While our earlier work did not observe differences in the number of live pups or in
Discussion
Several researchers have identified the GR as a key target for arsenic effects (Ahir et al., 2013, Barr et al., 2009, Bodwell et al., 2004, Gosse et al., 2014, Simons et al., 1990, Stancato et al., 1993). The GR pathway sits at a critical interaction and integration point to play a role in all of the key arsenic associated effects from: inflammation to cognition, and from immunity to neoplasia. Using a comparative toxicogemonics data base, Ahir and colleagues (Ahir et al., 2013) identified the
Conclusions
In our previous work assessing the impact of perinatal arsenic exposures on the function of the central and peripheral HPA system, we found significant support to conclude that arsenic exposure in utero resulted in a suppression of the glucocorticoid negative feedback system due to a decrease in GR and 11 HSD1 in the hippocampus. It was unclear if this deficit in HPA function was due to an indirect effect of arsenic resulting in a compensatory response to altered fetal HPA development or if the
Funding and disclosure
This work was supported by a grant from the National Institutes of Health: NIEHS RO1ES019583-01 (AMA). All authors declare no conflicts of interest.
The following are the Supplementary data related to this article.
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Acknowledgments
The authors thank Kevin Caldwell for helpful discussions and criticisms of the work. The authors received computer support from Michael Riblett. Dr. Abdulmehdi Ali performed the arsenic speciation analysis. The work was supported with funding 1RO1ES019583-01 (AMA) from the National Institute of Environmental Health Sciences.
Conceived and designed experiments: AMA and KEC. Performed experiments: all authors. Analyzed the data: all authors. Wrote the paper: all authors.
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