Expanded Systematic Evidence Map for Hundreds of Per- and Polyfluoroalkyl Substances (PFAS) and Comprehensive PFAS Human Health Dashboard.

BACKGROUND: Per- and polyfluoroalkyl substances (PFAS) encompass a class of chemically and structurally diverse compounds that are extensively used in industry and detected in the environment. The US Environmental Protection Agency (US EPA) 2021 PFAS Strategic Roadmap describes national research plans to address the challenge of PFAS. OBJECTIVES: Systematic Evidence Map (SEM) methods were used to survey and summarize available epidemiological and mammalian bioassay evidence that could inform human health hazard identification for a set of 345 PFAS that were identified by the US EPA's Center for Computational Toxicology and Exposure (CCTE) for in vitro toxicity and toxicokinetic assay testing and through interagency discussions on PFAS of interest. This work builds from the 2022 evidence map that collated evidence on a separate set of ∼150 PFAS. Like our previous work, this SEM does not include PFAS that are the subject of ongoing or completed assessments at the US EPA. METHODS: SEM methods were used to search, screen, and inventory mammalian bioassay and epidemiological literature from peer-reviewed and gray literature sources using manual review and machine-learning software. For each included study, study design details and health end points examined were summarized in interactive web-based literature inventories. Some included studies also underwent study evaluation and detailed extraction of health end point data. All underlying data is publicly available online as interactive visuals with downloadable metadata. RESULTS: More than 13,000 studies were identified from scientific databases. Screening processes identified 121 mammalian bioassay and 111 epidemiological studies that met screening criteria. Epidemiological evidence (available for 12 PFAS) mostly assessed the reproductive, endocrine, developmental, metabolic, cardiovascular, and immune systems. Mammalian bioassay evidence (available for 30 PFAS) commonly assessed effects in the reproductive, whole-body, nervous, and hepatic systems. Overall, 41 PFAS had evidence across mammalian bioassay and epidemiology data streams (roughly 11% of searched chemicals). DISCUSSION: No epidemiological and/or mammalian bioassay evidence were identified for most of the PFAS included in our search. Results from this SEM, our 2022 SEM on ∼150 PFAS, and other PFAS assessment products from the US EPA are compiled into a comprehensive PFAS dashboard that provides researchers and regulators an overview of the current PFAS human health landscape including data gaps and can serve as a scoping tool to facilitate prioritization of PFAS-related research and/or risk assessment activities. https://doi.org/10.1289/EHP13423.

Use of systematic evidence maps within the US environmental protection agency (EPA) integrated risk information system (IRIS) program: Advancements to date and looking ahead.

Systematic evidence maps (SEMs) are increasingly used to inform decision-making and risk management priority-setting and to serve as problem formulation tools to refine the focus of questions that get addressed in full systematic reviews. Within the U.S. Environmental Protection Agency (EPA) Office of Research and Development (ORD) Integrated Risk Information System (IRIS), SEMs have been used to inform data gaps, determine the need for updated assessments, inform assessment priorities, and inform development of study evaluation considerations, among other uses. Increased utilization of SEMs across the environmental health field has the potential to increase transparency and efficiency for data gathering, problem formulation, read-across, and evidence surveillance. Use of the SEM templates published in the companion text (Thayer et al.) can promote harmonization in the environmental health community and create more opportunities for sharing extracted content.

Systematic Evidence Map for Over One Hundred and Fifty Per- and Polyfluoroalkyl Substances (PFAS).

BACKGROUND: Per- and polyfluoroalkyl substances (PFAS) are a large class of synthetic (man-made) chemicals widely used in consumer products and industrial processes. Thousands of distinct PFAS exist in commerce. The 2019 U.S. Environmental Protection Agency (U.S. EPA) Per- and Polyfluoroalkyl Substances (PFAS) Action Plan outlines a multiprogram national research plan to address the challenge of PFAS. One component of this strategy involves the use of systematic evidence map (SEM) approaches to characterize the evidence base for hundreds of PFAS. OBJECTIVE: SEM methods were used to summarize available epidemiological and animal bioassay evidence for a set of ∼150 PFAS that were prioritized in 2019 by the U.S. EPA's Center for Computational Toxicology and Exposure (CCTE) for in vitro toxicity and toxicokinetic assay testing. METHODS: Systematic review methods were used to identify and screen literature using manual review and machine-learning software. The Populations, Exposures, Comparators, and Outcomes (PECO) criteria were kept broad to identify mammalian animal bioassay and epidemiological studies that could inform human hazard identification. A variety of supplemental content was also tracked, including information on in vitro model systems; exposure measurement-only studies in humans; and absorption, distribution, metabolism, and excretion (ADME). Animal bioassay and epidemiology studies meeting PECO criteria were summarized with respect to study design, and health system(s) were assessed. Because animal bioassay studies with ≥21-d exposure duration (or reproductive/developmental study design) were most useful to CCTE analyses, these studies underwent study evaluation and detailed data extraction. All data extraction is publicly available online as interactive visuals with downloadable metadata. RESULTS: More than 40,000 studies were identified from scientific databases. Screening processes identified 44 animal and 148 epidemiology studies from the peer-reviewed literature and 95 animal and 50 epidemiology studies from gray literature that met PECO criteria. Epidemiological evidence (available for 15 PFAS) mostly assessed the reproductive, endocrine, developmental, metabolic, cardiovascular, and immune systems. Animal evidence (available for 40 PFAS) commonly assessed effects in the reproductive, developmental, urinary, immunological, and hepatic systems. Overall, 45 PFAS had evidence across animal and epidemiology data streams. DISCUSSION: Many of the ∼150 PFAS were data poor. Epidemiological and animal evidence were lacking for most of the PFAS included in our search. By disseminating this information, we hope to facilitate additional assessment work by providing the initial scoping literature survey and identifying key research needs. Future research on data-poor PFAS will help support a more complete understanding of the potential health effects from PFAS exposures. https://doi.org/10.1289/EHP10343.

A case study on the application of an expert-driven read-across approach in support of quantitative risk assessment of p,p'-dichlorodiphenyldichloroethane.

Deriving human health risk estimates for environmental chemicals has traditionally relied on in vivo toxicity databases to characterize potential adverse health effects and associated dose-response relationships. In the absence of in vivo toxicity information, new approach methods (NAMs) such as read-across have the potential to fill the required data gaps. This case study applied an expert-driven read-across approach to identify and evaluate analogues to fill non-cancer oral toxicity data gaps for p,p'-dichlorodiphenyldichloroethane (p,p'-DDD), an organochlorine contaminant known to occur at contaminated sites in the U.S. The source analogue p,p'-dichlorodiphenyltrichloroethane (DDT) and its no-observed-adverse-effect level of 0.05 mg/kg-day were proposed for the derivation of screening-level health reference values for the target chemical, p,p'-DDD. Among the primary similarity contexts (structure, toxicokinetics, and toxicodynamics), toxicokinetic considerations were instrumental in separating p,p'-DDT as the best source analogue from other potential candidates (p,p'-DDE and methoxychlor). In vitro high-throughput screening (HTS) assays from ToxCast were used to evaluate similarity in bioactivity profiles and make inferences toward plausible mechanisms of toxicity to build confidence in the read-across approach. This work demonstrated the value of NAMs such as read-across and in vitro HTS in human health risk assessment of environmental contaminants with the potential to inform regulatory decision-making.

Epigenetic Regulation in Environmental Chemical Carcinogenesis and its Applicability in Human Health Risk Assessment.

Although several studies have shown that chemically mediated epigenetic changes are an etiological factor in several human disease conditions, the utility of epigenetic data, such as DNA methylation, in the current human health risk assessment paradigm is unclear. The objective of this study is to investigate the relationship between the points of departure (PODs) for cancer incidence and DNA methylation changes in laboratory animals exposed to the following environmental toxicants: bromodichloromethane, dibromochloromethane, chloroform, hydrazine, trichloroethylene, benzidine, trichloroacetic acid, and di(2-ethylhexyl) phthalate (DEHP; a known reproductive toxicant). The results demonstrate that the PODs for cancer incidence and altered DNA methylation are similar. Furthermore, based on the available data, the POD for DNA methylation appeared more sensitive compared to that for cancer incidence following the administration of DEHP to rats during different life stages. The high degree of correlation between PODs for cancer incidence and DNA methylation (for both total DNA and individual genes) suggests that DNA methylation end points could potentially be used as a screening tool in predicting the potential toxicity/carcinogenicity and in prioritizing large numbers of chemicals with sparse toxicity databases. The life stage during which treatment occurs is also an important consideration when assessing the potential application of epigenetic end points as a screening tool.

PKCε contributes to chronic ethanol-induced steatosis in mice but not inflammation and necrosis.

BACKGROUND: Protein kinase C epsilon (PKCε) has been shown to play a role in experimental steatosis by acute alcohol. The "two-hit" hypothesis implies that preventing steatosis should blunt more advanced liver damage (e.g., inflammation and necrosis). However, the role of PKCε in these pathologies is not yet known. The goal of this current work was to address this question in a model of chronic alcohol exposure using antisense oligonucleotides (ASO) against PKCε. METHODS: Accordingly, PKCε ASO- and saline-treated mice were fed high-fat control or ethanol (EtOH)-containing enteral diets for 4 weeks. RESULTS: Chronic EtOH exposure significantly elevated hepatic lipid pools as well as activated PKCε. The PKCε ASO partially blunted the increases in hepatic lipids caused by EtOH. Administration of PKCε ASO also completely prevented the increase in the expression of fatty acid synthase, and tumor necrosis factor α caused by EtOH. Despite these protective effects, the PKCε ASO was unable to prevent the increases in inflammation and necrosis caused by chronic EtOH. These latter results correlated with an inability of the PKCε ASO to blunt the up-regulation of plasminogen activator inhibitor-1 (PAI-1) and the accumulation of fibrin. Importantly, PAI-1 has been previously shown to more robustly mediate inflammation and necrosis (vs. steatosis) after chronic EtOH exposure. CONCLUSIONS: This study identifies a novel potential mechanism where EtOH, independent of steatosis, can contribute to liver damage. These results also suggest that PAI-1 and fibrin accumulation may be at the center of this PKCε-independent pathway.

Putative mechanisms of environmental chemical-induced steatosis.

Liver disease is a major health issue characterized by several pathological changes, with steatosis (fatty liver) representing a common initial step in its pathogenesis. Steatosis is of critical importance because prevention of fatty liver can obviate downstream pathologies of liver disease (eg, fibrosis). Recent studies have shown a strong correlation between chemical exposure and steatosis. The work described here identifies chemicals on the US Environmental Protection Agency's Integrated Risk Information System (IRIS) that induce steatosis and investigates putative mechanisms by which these chemicals may contribute to this pathological condition. Mitochondrial impairment, insulin resistance, impaired hepatic lipid secretion, and enhanced cytokine production were identified as potential mechanisms that could contribute to steatosis. Taken together, this work is significant because it identifies multiple mechanisms by which environmental chemicals may cause fatty liver and expands our knowledge of the possible role of environmental chemical exposure in the induction and progression of liver disease.

Plasminogen activator inhibitor-1 deficient mice are protected from angiotensin II-induced fibrosis.

PAI-1 has been shown to be both profibrotic and antifibrotic in animal models of hepatic fibrosis. Although these models have similarities to human fibrotic liver disease, no rodent model completely recapitulates the clinical situation; indeed, transaminase values in most models of hepatic fibrosis are much higher than in chronic liver diseases in humans. Here, wild-type and PAI-1(-/-) mice were administered AngII (500 ng/kg/min) for 4 weeks. ECM accumulation was evaluated by Sirius red staining, hydroxyproline content, and fibrin and collagen immunostaining. Induction of pro-fibrotic genes was assessed by real-time RT-PCR. Despite the absence of any significant liver damage, AngII infusion increased the deposition of hepatic collagen and fibrin ECM, with a perisinusoidal pattern. PAI-1(-/-) mice were protected from these ECM changes, indicating a causal role of PAI-1 in this fibrosis model. Protection in the knockout strain correlated with a blunted increase in αSMA, and elevated activities of matrix metalloproteinases (MMP2, MMP9). These data suggest that PAI-1 plays a critical role in mediating fibrosis caused by AngII and lends weight-of-evidence to a pro-fibrotic role of this protein in liver. Furthermore, the current study proposes a new model of 'pure' hepatic fibrosis in mice with little inflammation or hepatocyte death.

PAI-1 plays a protective role in CCl4-induced hepatic fibrosis in mice: role of hepatocyte division.

Plasminogen activator inhibitor-1 (PAI-1) is an acute phase protein that has been shown to play a role in experimental fibrosis caused by bile duct ligation (BDL) in mice. However, its role in more severe models of hepatic fibrosis (e.g., carbon tetrachloride; CCl(4)) has not been determined and is important for extrapolation to human disease. Wild-type or PAI-1 knockout mice were administered CCl(4) (1 ml/kg body wt ip) 2x/wk for 4 wk. Plasma (e.g., transaminase activity) and histological (e.g., Sirius red staining) indexes of liver damage and fibrosis were evaluated. Proliferation and apoptosis were assessed by PCNA and TdT-mediated dUTP nick-end labeling (TUNEL) staining, respectively, as well as by indexes of cell cycle (e.g., p53, cyclin D1). In contrast to previous studies with BDL, hepatic fibrosis was enhanced in PAI-1(-/-) mice after chronic CCl(4) administration. Indeed, all indexes of liver damage were elevated in PAI-1(-/-) mice compared with wild-type mice. This enhanced liver damage correlated with impaired hepatocyte proliferation. A similar effect on proliferation was observed after one bolus dose of CCl(4), without concomitant increases in liver damage. Under these conditions, a decrease in phospho-p38, coupled with elevated p53 protein, was observed; these results suggest impaired proliferation and a potential G(1)/S cell cycle arrest in PAI-1(-/-) mice. These data suggest that PAI-1 may play multiple roles in chronic liver diseases, both protective and damaging, the latter mediated by its influence on inflammation and fibrosis and the former via helping maintain hepatocyte division after an injury.

PKCepsilon plays a causal role in acute ethanol-induced steatosis.

Steatosis is a critical stage in the pathology of alcoholic liver disease (ALD), and preventing steatosis could protect against later stages of ALD. PKCepsilon has been shown to contribute to hepatic steatosis in experimental non-alcoholic fatty liver disease (NAFLD); however, the role of PKCepsilon in ethanol-induced steatosis has not been determined. The purpose of this study was to therefore test the hypothesis that PKCepsilon contributes to ethanol-induced steatosis. Accordingly, the effect of acute ethanol on indices of hepatic steatosis and insulin signaling were determined in PKCepsilon knockout mice and in wild-type mice that received an anti-sense oligonucleotide (ASO) to knockdown PKCepsilon expression. Acute ethanol (6g/kg i.g.) caused a robust increase in hepatic non-esterified free fatty acids (NEFA), which peaked 1h after ethanol exposure. This increase in NEFA was followed by elevated diacylglycerols (DAG), as well as by the concomitant activation of PKCepsilon. Acute ethanol also changed the expression of insulin-responsive genes (i.e. increased G6Pase, downregulated GK), in a pattern indicative of impaired insulin signaling. Acute ethanol exposure subsequently caused a robust increase in hepatic triglycerides. The accumulation of triglycerides caused by ethanol was blunted in ASO-treated or in PKCepsilon(-/-) mice. Taken together, these data suggest that the increase in NEFA caused by hepatic ethanol metabolism leads to an increase in DAG production via the triacylglycerol pathway. DAG then subsequently activates PKCepsilon, which then exacerbates hepatic lipid accumulation by inducing insulin resistance. These data also suggest that PKCepsilon plays a causal role in at least the early phases of ethanol-induced liver injury.

New role of resistin in lipopolysaccharide-induced liver damage in mice.

Studies in rodents suggest that the adipocytokine resistin causes insulin resistance via impairing normal insulin signaling. However, in humans, resistin may play a more important role in inflammation than in insulin resistance. Whether resistin contributes to inflammation in rodents is unclear. Therefore, the purpose of the present study was to determine the effect of resistin exposure on the basal and stimulated [lipopolysaccharide (LPS)] inflammatory response in mouse liver in vivo. Resistin alone had no major effects on hepatic expression of insulin-responsive genes, either in the presence or absence of LPS. Although it had no effect alone, resistin significantly enhanced hepatic inflammation and necrosis caused by LPS. Resistin increased expression of proinflammatory genes, e.g., plasminogen activator inhibitor (PAI)-1, and activity of mitogen-activated protein (MAP) kinase, extracellular signal-regulated kinase 1/2, caused by LPS, but had little effect on anti-inflammatory gene expression. Resistin also enhanced fibrin deposition (an index of hemostasis) caused by LPS. The increase in PAI-1 expression, fibrin deposition, and liver damage caused by LPS + resistin was almost completely prevented either by inhibiting the coagulation cascade, hirudin, or by blocking MAP kinase signaling, U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio) butadiene], indicating that these pathways play a causal role in observed enhanced liver damage caused by resistin. Taken together, the augmentation of LPS-induced liver damage caused by resistin seems to involve, at least in part, up-regulation of hepatic inflammation via mechanisms most likely involving the coagulation cascade and fibrin accumulation. These data also suggest that resistin may have proinflammatory roles in mouse liver independent of its effects on insulin signaling, analogous to previous work in humans.

Contribution of the sympathetic hormone epinephrine to the sensitizing effect of ethanol on LPS-induced liver damage in mice.

It is well known that ethanol preexposure sensitizes the liver to LPS hepatotoxicity. The mechanisms by which ethanol enhances LPS-induced liver injury are not completely elucidated but are known to involve an enhanced inflammatory response. Ethanol exposure also increases the metabolic rate of the liver, and this effect of ethanol on liver is mediated, at least in part, by the sympathetic hormone, epinephrine. However, whether or not the sympathetic nervous system also contributes to the sensitizing effect of ethanol preexposure on LPS-induced liver damage has not been determined. The purpose of this study was therefore to test the hypotheses that 1) epinephrine preexposure enhances LPS-induced liver damage (comparable to that of ethanol preexposure) and that 2) the sympathetic nervous system contributes to the sensitizing effect of ethanol. Accordingly, male C57BL/6J mice were administered epinephrine for 5 days (2 mg/kg per day) via osmotic pumps or bolus ethanol for 3 days (6 g/kg per day) by gavage. Twenty-four hours later, mice were injected with LPS (10 mg/kg ip). Both epinephrine and ethanol preexposure exacerbated LPS-induced liver damage and inflammation. Concomitant administration of propranolol with ethanol significantly attenuated the sensitizing effect of ethanol on LPS-induced liver damage. These data support the hypothesis that the sympathetic nervous system contributes, at least in part, to the mechanism of the sensitizing effect of ethanol. These results also suggest that sympathetic tone may contribute to the initiation and progression of alcoholic liver disease.

Subhepatotoxic exposure to arsenic enhances lipopolysaccharide-induced liver injury in mice.

Exposure to arsenic via drinking water is a serious health concern in the US. Whereas studies have identified arsenic alone as an independent risk factor for liver disease, concentrations of arsenic required to damage this organ are generally higher than found in the US water supply. The purpose of the current study was to test the hypothesis that arsenic (at subhepatotoxic doses) may also sensitize the liver to a second hepatotoxin. To test this hypothesis, the effect of chronic exposure to arsenic on liver damage caused by acute lipopolysaccharide (LPS) was determined in mice. Male C57Bl/6J mice (4-6 weeks) were exposed to arsenic (49 ppm as sodium arsenite in drinking water). After 7 months of exposure, animals were injected with LPS (10 mg/kg i.p.) and sacrificed 24 h later. Arsenic alone caused no overt hepatotoxicity, as determined by plasma enzymes and histology. In contrast, arsenic exposure dramatically enhanced liver damage caused by LPS, increasing the number and size of necroinflammatory foci. This effect of arsenic was coupled with increases in indices of oxidative stress (4-HNE adducts, depletion of GSH and methionine pools). The number of apoptotic (TUNEL) hepatocytes was similar in the LPS and arsenic/LPS groups. In contrast, arsenic pre-exposure blunted the increase in proliferating (PCNA) hepatocytes caused by LPS; this change in the balance between cell death and proliferation was coupled with a robust loss of liver weight in the arsenic/LPS compared to the LPS alone group. The impairment of proliferation after LPS caused by arsenic was also coupled with alterations in the expression of key mediators of cell cycle progression (p27, p21, CDK6 and Cyclin D1). Taken together, these results suggest that arsenic, at doses that are not overtly hepatotoxic per se, significantly enhances LPS-induced liver injury. These results further suggest that arsenic levels in the drinking water may be a risk modifier for the development of chronic liver diseases.