The following summarizes the presentations given during the Platform Session “Mixtures Toxicology” that was held as part of the 2023 SOT Annual Meeting and ToxExpo.
Association between Exposure to Toxic and Essential Metal Mixtures and Gestational Age in the ELGAN Cohort
Presenter: Eric Brown Jr., University of North Carolina at Chapel Hill
Nearly 15 million infants in the US are born preterm, which equates to nearly 10% of annual births. Toxic metals are associated with decreased gestational age, while essential metals have a protective effect. Utilizing the Extremely Low Gestational Age Newborn (ELGAN) cohort, the group set out to investigate the associations between umbilical cord blood metal concentrations—singularly and as a mixture—and gestational age. The umbilical cord blood was evaluated for the content of 11 metals. In covariate-adjusted single-pollutant analyses, strontium and selenium were significantly associated with lower gestational age. In mixtures analyses, the total mixture was associated with a reduced gestational age at birth. Interestingly, some metals (lead, copper, antimony, and arsenic) were associated with a higher gestational age, while others (barium, manganese, selenium, cadmium, mercury, zinc, and strontium) were associated with a reduced gestational age. Overall, mixtures of metals were associated with decreased gestation, which may have implications for maternal and fetal health.
A Mixture of Epidermal Growth Factor Receptor-Disrupting Chemicals Reduce Cellular Bioenergetics and Alter Mitochondrial Dynamics in Human Primary Cytotrophoblast Cells
Presenter: Elvis Ticiani, University of Illinois at Chicago
Healthy placenta are vital for pregnancy health and require functional mitochondria to maintain pregnancy. However, pregnant women are exposed to complex environmental mixtures that can traverse but also target the placenta. The epidermal growth factor receptor (EGFR) is highly expressed in the placenta, and the EGF ligand is important for stimulating mitochondrial respiration in trophoblasts. Given the importance of energy production in pregnancy, using isolated cytotrophoblasts from human placentas, the lab investigated the question, “Do EGFR-disrupting chemicals impact trophoblast bioenergetics?” Mitochondrial “stress tests” were performed to evaluate basal and maximum respiratory capacity, and it was found that the chemical mixture reduced the maximum respiratory capacity of cells (oxygen consumption) in a dose-dependent manner. Then, to determine if this effect was EGFR-mediated, cells were exposed to a chemical mixture with or without EGF. Overall, the chemical mixture reduced both EGF- and non-EGF-mediated oxygen consumption rates and ATP production of cells. Finally, whether changes in oxygen consumption and adenosine triphosphate production were due to mitochondrial remodeling was investigated. Using special visualization tools and evaluation of gene expression, the lab found that exposure to a chemical mixture reduced the mitochondrial network and the expression in OPA1, an important gene involved in mitochondrial fusion. Thus, EGFR-disrupting chemicals alter mitochondrial remodeling, which has implications for cellular respiration and mitochondrial function.
Role of Alkylated Polycyclic Aromatic Hydrocarbons in Mixture Toxicity from a Legacy Creosote Site
Presenter: Ian Moran, Oregon State University
Creosote is a pesticide and complex mixture of polycyclic aromatic hydrocarbons (PAHs) that has traditionally been used to preserve wood. Despite its ubiquity, the toxic effects of creosote are largely unknown. The goal of the study was to understand and characterize the contribution of alkylated PAHs to the toxicity of a complex mixture from a legacy creosote site. To obtain creosote, low-density polyethylene passive samplers were utilized at a closed wood treatment facility, where creosote had dissolved into the surrounding water. Samplers were analyzed by gas chromatography for unsubstituted and alkylated PAHs. Exposed embryonic zebrafish were utilized to evaluate toxicity, specifically behavioral and morphological endpoints. The goal of this was to allow the biological systems (within zebrafish) to determine which outcomes may be important. Finally, fractionation of field collected mixtures were used to determine drivers of toxicity. Fractionation by gel permeation chromatography revealed that three fractions together recapitulated the toxicity of the whole mixture. The fraction containing the alkylated PAHs caused most of the toxicity. A third unidentified fraction caused high incidence of notochord malformation in zebrafish at low concentrations but no lethal effects. In summary, alkylated PAHs released from creosote may have implications for human health and thus need to be further investigated.
Mammalian Toxicity of Environmental Debris and Plastic Particle Mimetics
Presenter: Sarah Morgan, University of Rochester Medical Center
Microplastics are ubiquitous particles ranging from one micrometer to five millimeters to which humans are inevitably exposed. Animal models are limited in their ability to help us understand human exposure, given that the impact of microplastics on mortality and reproduction vary by the taxonomic group studied. Additionally, environmental microplastics differ from those found in the laboratory setting. It is likely that microplastic mixtures produce different toxicity effects than individual exposures. To test the effects of a microplastic mixture, the lab took two approaches: one using environmental particles and another using plastic particle mimetics. In analyses using the environmental microplastic mixture collected from Lake Ontario, it was observed that the debris increased aryl hydrocarbon receptor (AhR) activity but was not cytotoxic in human keratinocyte (HaCaT) cells. When using the plastic particle mimetics, specifically microbeads and pollutants (polystyrene [PS], tetrachlorodibenzodioxin [TCDD], and bisphenol A [BPA]), it appeared that the beads absorbed TCDD which decreased AhR activity. However, BPA exposure did inhibit AhR activity but adding PS inhibited BPA’s inhibition of AhR activity. This highlights the need for a better understanding of microplastics and their interactions with other chemicals and what this means for human health.
PFAS Mixtures: RPFs or Sums?
Presenter: Thomas Webster, Boston University School of Public Health
Per- and polyfluoroalkyl substances (PFAS) are environmentally persistent chemicals that are generally present in complex mixtures. Specifically, there are over 10,000 PFAS, and toxicologic data are sparse or absent for most. The United States and other countries have attempted to regulate PFAS limits in the water and environment, but they have taken a “sum” approach, wherein the limit is set based on the health effects of several well-known PFAS. The European Union has proposed a different approach, using the relative potency factors (RPFs) model, which requires a dose-response curve that only differs in potency (all lines will be parallel). Additionally, it must be experimentally shown that the PFAS (or other chemicals of interest) demonstrate concentration addition. Finally, the RPFs should apply to all outcomes of interest. Unfortunately, experimental models have indicated that it is unclear that the dose-response curves have the same efficacy for each PFAS and that they do not demonstrate concentration addition. Furthermore, research from the Webster lab shows that there are clear differences in efficacy of PPAR-alpha activation by PFAS in a reporter assay. As a result, PFAS mixtures data fit concentration addition better than the RPFs for PPAR-alpha, which overestimate the true effect. PPAR-alpha activation is thought to be an important molecular initiating event (MIE) for PFAS, but there seem to be several other receptors activated by PFAS in the liver. RPFs fit the one MIE situation very well. All the PFAS will interact with the one receptor (the single MIE) that has multiple outcomes. However, PFAS likely interact with multiple MIEs, which also means that there are multiple outcomes and it is then difficult to predict mixtures effects. One study recently observed that mixtures using PFOA and PFOS were dose-additive for many maternal and neonatal effects of exposed rats in pregnancy and that RPF models worked well but depended on the outcome. Overall, there are several considerations for the use of RPFs, and outcome-specific concentrations could be problematic. However, there is a possible reconciliation using generalized concentration addition.
Mapping a Path to Disease: Quantifying the Risk of Exposure to Environmental Chemical Mixtures via a Common Molecular Target Using a Geospatial Modeling Approach
Presenter: Kristin Eccles, NIEHS
Risk assessments conducted in vivo generally use a chemical-by-chemical approach, but there are many chemicals on which data is severely lacking. Furthermore, some chemicals also are missing data on apical endpoints, which are usually of interest in most studies. Using geospatial methods is a model way to conduct risk assessment based on the chemical perturbations of a molecular target. Thus, the aim of this study was to identify regions in the US where chemical mixtures may disrupt the same biological target(s). To achieve this goal, county-level air exposure was modeled using the National Air Toxics Assessment, combined with hazard data evaluating CYP1A1 expression. Physiologically based toxicokinetic modeling was employed to convert inhaled chemical concentrations into plasma concentrations while accounting for age and obesity status because these are important factors involved in chemical clearance. The risk of biological perturbation was mapped by county, and there was a heterogeneous distribution of risk; chemical cooccurrence did not predict risk. Using a Monte-Carlo uncertainty analysis, it was found that the factors most affecting risk of perturbation were external concentrations and concentration-response parameters. This method can be expanded to apply to other chemicals, routes of exposure, and molecular targets, thus advancing chemical risk assessment and assessment of adverse health outcomes.
This blog reports on the Platform Session titled “Mixtures Toxicology” that was held during the 2023 SOT Annual Meeting and ToxExpo. An on-demand recording of this session is available for meeting registrants on the SOT Online Planner and SOT Event App.
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