Evaluation of neural reflex activation as a potential mode of action for respiratory and cardiovascular effects of fine particulate matter.

OBJECTIVES: Mortality from respiratory and cardiovascular health conditions contributes largely to the total mortality that has been associated with exposure to PM2.5 in epidemiology studies. A mode of action (MoA) for these underlying morbidities has not been established, but it has been proposed that some effects of PM2.5 occur through activation of neural reflexes. MATERIALS AND METHODS: We critically reviewed the experimental studies of PM2.5 (including ambient PM2.5, diesel exhaust particles, concentrated ambient particles, diesel exhaust, and cigarette smoke) and neural reflex activation, and applied the principles of the International Programme on Chemical Safety (IPCS) MoA/human relevance framework to assess whether they support a biologically plausible and human-relevant MoA by which PM2.5 could contribute to cardiovascular and respiratory causes of death. We also considered whether the evidence from these studies supports a non-threshold MoA that operates at low, human-relevant PM2.5 exposure concentrations. RESULTS AND DISCUSSION: We found that the proposed MoA of neural reflex activation is biologically plausible for PM2.5-induced respiratory effects at high exposure levels used in experimental studies, but further studies are needed to fill important data gaps regarding the relevance of this MoA to humans at lower PM2.5 exposure levels. A role for the proposed MoA in PM2.5-induced cardiovascular effects is plausible for some effects but not others. CONCLUSIONS: Further studies are needed to determine whether neural reflex activation is the MoA by which PM2.5 could cause either respiratory or cardiovascular morbidities in humans, particularly at the ambient concentrations associated with total mortality in epidemiology studies.

High-throughput screening of toxicants that modulate extravillous trophoblast migration.

Migration and subsequent invasion of extravillous trophoblasts into the uterus is essential for proper formation of the placenta. Disruption of these processes may result in poor pregnancy outcomes including preeclampsia, placenta accreta, fetal growth restriction, or fetal death. Currently, there are several methods for quantifying cell migration and invasion in vitro, each with limitations. Therefore, we developed a novel, high-throughput method to screen chemicals for their ability to alter human trophoblast migration. Human HTR8/SVneo trophoblast cells were cultured in Oris™ cell migration plates containing stopper barriers. After EVT cells attached and chemicals were added to media, stoppers were removed thereby creating a cell-free detection zone for migration. Entry of trophoblasts into this zone was monitored through imaging every 6 h and used to calculate a relative cell density. Chemicals known to increase (epidermal growth factor) and decrease (pertussis toxin and cadmium) trophoblast migration were used to validate this in vitro method. Next, a panel of environmental chemicals including bisphenols, mycoestrogens, and flame retardants, were screened for their ability to alter trophoblast invasion. In conclusion, a real-time method to track extravillous trophoblast migration offers potential for screening contaminants as placental toxicants.

Extravillous trophoblast migration and invasion: Impact of environmental chemicals and pharmaceuticals.

During pregnancy, the migration and invasion of extravillous trophoblasts (EVTs) into the maternal uterus is essential for proper development of the placenta and fetus. During the first trimester, EVTs engraft and remodel maternal spiral arteries allowing for efficient blood flow and the transfer of essential nutrients and oxygen to the fetus. Aberrant migration of EVTs leading to either shallow or deep invasion into the uterus has been implicated in a number of gestational pathologies including preeclampsia, fetal growth restriction, and placenta accreta spectrum. The migration and invasion of EVTs is well-coordinated to ensure proper placentation. However, recent data point to the ability of xenobiotics to disrupt EVT migration. These xenobiotics include heavy metals, endocrine disrupting chemicals, and organic contaminants and have often been associated with adverse pregnancy outcomes. In most instances, xenobiotics appear to reduce EVT migration; however, there are select examples of enhanced motility after chemical exposure. In this review, we provide an overview of the 1) current experimental approaches used to evaluate EVT migration and invasion in vitro, 2) ability of environmental chemicals and pharmaceuticals to enhance or retard EVT motility, and 3) signaling pathways responsible for altered EVT migration that are sensitive to disruption by xenobiotics.

Inorganic arsenic and its methylated metabolites as endocrine disruptors in the placenta: Mechanisms underpinning glucocorticoid receptor (GR) pathway perturbations.

Exposure to inorganic arsenic (iAs) is a significant public health concern with individuals around the globe exposed to harmful levels through contaminated drinking water. Exposure to iAs during pregnancy is of particular concern and has been associated with pregnancy complications and adverse child health later in life. Effects of in utero exposure may be mediated through alterations in key signaling pathways in the placenta that regulate fetal growth and development. A pathway of interest is the glucocorticoid receptor (GR)- signaling pathway, which is known to regulate fetal and placental development. While prior research has shown that iAs alters GR-associated gene expression in trophoblasts, the mechanisms that underlie these perturbations remain unknown. In the present study, we set out to elucidate the molecular mechanisms that underpin observed alterations in GR-associated gene expression. We also aimed to determine whether the methylated metabolites of iAs, namely monomethyl­arsenic (MMA) and dimethyl­arsenic (DMA), also influence GR-associated signaling in the placenta. The data indicate that iAs alters GR activation in a dose-dependent manner, reduces nuclear translocation, and reduces DNA binding. Additionally, the results demonstrate that MMA and DMA alter the expression of eight GR-associated genes, modulate GR activation, and alter DNA binding. These data are significant as they highlight the role of iAs as an endocrine disruptor and for the first time explore the effects of MMA and DMA on endocrine signaling in the placenta.

Inorganic Arsenic as an Endocrine Disruptor: Modulation of the Glucocorticoid Receptor Pathway in Placental Cells via CpG Methylation.

Prenatal exposure to inorganic arsenic (iAs) has been associated with adverse developmental and reproductive outcomes. These outcomes may be tied to altered functionality of nuclear transcription factors such as the glucocorticoid receptor (GR) in the placenta and associated gene expression. The GR pathway is integral for proper fetal and placental development, and perturbations in this pathway may underlie observed associations between prenatal iAs exposure and adverse birth outcomes. We therefore set out to investigate whether iAs modulates the GR signaling pathway in placental cells. JEG-3 trophoblasts were exposed to environmentally-relevant doses of iAs, and mRNA expression assessed. To examine the links between iAs exposure, the GR signaling pathway, and epigenetic modification, DNA methylation levels were also quantified. Treatment with iAs altered the expression of 12 GR-genes that play a role in fetal and placental development. Furthermore, at a gene-specific level, mRNA abundance was associated with changes in DNA methylation patterning in JEG-3 cells, suggesting that the effects of iAs are mediated by epigenetic mechanisms. The identified target genes have been associated with prenatal iAs exposure, placental physiology, and fetal development. This study provides further evidence for iAs as an endocrine disruptor and provides insight as to the mechanisms by which prenatal iAs exposure may induce adverse birth outcomes.