Test driving ToxCast: endocrine profiling for 1858 chemicals included in phase II

doi:10.1016/j.coph.2014.09.021

Current Opinion in Pharmacology

Volume 19, December 2014, Pages 145-152

https://doi.org/10.1016/j.coph.2014.09.021 Get rights and content

Highlights

•
1858 chemicals in EPA's ToxCast program were analyzed for endocrine assay targets.
•
ToxPi model utilizes 85 in vitro HTS assays and 2 physicochemical properties.
•
We focused on estrogen, androgen, thyroid, glucocorticoid, PPARs and monoamines.
•
Many chemicals showing activity in the in vitro assays have not been studied for potential endocrine disruption.
•
Additional research is required to confirm these screening level analyses.

Identifying chemicals, beyond those already implicated, to test for potential endocrine disruption is a challenge and high throughput approaches have emerged as a potential tool for this type of screening. This review focused the Environmental Protection Agency's (EPA) ToxCast™ high throughput in vitro screening (HTS) program. Utility for identifying compounds was assessed and reviewed by using it to run the recently expanded chemical library (from 309 compounds to 1858) through the ToxPi™ prioritization scheme for endocrine disruption. The analysis included metabolic and neuroendocrine targets. This investigative approach simultaneously assessed the utility of ToxCast, and helped identify novel chemicals which may have endocrine activity. Results from this exercise suggest the spectrum of environmental chemicals with potential endocrine activity is much broader than indicated, and that some aspects of endocrine disruption are not fully covered in ToxCast.

Introduction

Endocrine disrupting chemicals (EDCs) are exogenous chemicals, or mixture of chemicals, that can interfere with any aspect of hormone action [1]. Many of these substances have been linked with developmental, reproductive, neural, immune, and other problems in wildlife and laboratory animals [2^•]. Most environmental chemicals have never been fully assessed for potential endocrine activity, thus information regarding how many EDCs exists in our environment, how humans are exposed, and to what degree they post a risk to human health is limited [2^•]. As the largest funding agency for EDC research, the National Institute of Environmental Health Sciences (NIEHS) has tended to focus on a relatively small number of chemicals such as bisphenols, phthalates, halogenated persistent compounds, genistein and other isoflavones, and certain pesticides (Figure 1). This focus on a relatively few chemicals is partly due to a lack of research tools to help identify and prioritize which chemicals to study based on indications of potential endocrine activity. Moreover, it can be very difficult to obtain funding support to generate descriptive information on data-poor chemicals in the absence of a specific hypothesis to test. Because chemicals can enter the marketplace (with some exceptions) without any toxicity information, researchers typically have nothing to go on to develop hypotheses.

High throughput screening approaches within the toxicological community hope to help identify chemicals with potential endocrine activity. The USEPA ToxCast program has completed Phase II screening efforts of 1858 unique chemicals in hundreds of HTS assays [3^•] (Figure 2). Screening programs like ToxCast are a major component of modern toxicology, necessitated by the recognition that animal studies alone are insufficient to generate data on thousands of chemicals in terms of cost, time, and throughput, Previously, Reif et al. [4^••] developed a Toxicological Priority Index (ToxPi) model to describe and prioritize 309 unique chemicals for potential endocrine activity using Phase I ToxCast data. ToxPi is an information graphic used to display information on a chemical's bioactivity profiles, inferred toxicity pathways, dose estimates, exposure data, chemical structural descriptors, or other features [5]. The purpose of this article is twofold. First, we use state of the art approaches for analyzing HTS data to identify environmental chemicals with potential endocrine activity. We did this by updating the previous prioritization scheme with Phase II ToxCast data, covering an additional 1549 chemicals. In addition to expanding the chemical coverage, we also made minor changes to the model methodology and included several other EDC assay targets of growing interest in environmental health. The updated prioritization scheme focuses on the classic targets of the estrogen, androgen, and thyroid pathways as well as targets of emerging interest such as the glucocorticoid receptor, peroxisome proliferator-activated receptors (PPARs), and monoamine signaling. Second, through this exercise, we review the strengths and weaknesses of the ToxPi tool, and identify potential new endocrine disruptors which might be an important focus of future research efforts. Thus, this review both summarizes and enhances ToxPi-based screens of ToxCast chemicals for endocrine disrupting chemicals and advocates for engagement and further work on strengthening screening and prioritization strategies.

Section snippets

Publicly available ToxCast data

ToxCast currently provides results from 821 assay endpoints that make use of numerous technology platforms from seven vendors [3^•] (Figure 2). These platforms include both cell-free (biochemical) and cell-based measures in multiple human primary cells, human or rodent cell lines, and rat primary hepatocytes [6]. A wide spectrum of biological targets or effects is covered, including cytotoxicity, cell growth, genotoxicity, enzymatic activity, receptor binding, reporter gene activity (mostly

Updates to ToxPi prioritization scheme

The methodology used in this review builds on previous work by Reif et al. [4^••] where 309 chemicals tested in Phase 1 of ToxCast were profiled for potential estrogen, androgen, and thyroid disruption. In this review we expand the original prioritization scheme by summarizing the currently available data for the larger 1858 chemical ToxCast library. In addition to traditional endocrine disruption targets, we highlighted an expanded list of endocrine-related targets to include areas of emerging

Updated EDC ToxPi findings

Figure 3 summarizes one approach for identifying some of the most active environmental compounds. We show the ToxPi profiles for the 30 top scoring chemicals, excluding pharmaceutical and endogenous compounds. The list includes chemicals with specific profiles, like zearalenone and bisphenol AF showing potential estrogenic activity, and chemicals like apigenin with less specific profiles. We also demonstrate an alternative strategy where we identified the top scoring chemicals for each specific

Conclusions

We encourage others to use the public ToxCast data to explore their own hypotheses about specific axes of toxicological hazard probed by this diverse assay set. Here, we have explored one such axis, endocrine activity, by building a ToxPi prioritization scheme from relevant ToxCast assays and placing the results in context with reference chemicals having known EDC activity. Information on the assays can be accessed through the iCSS Dashboards (http://actor.epa.gov/dashboard/), which includes

Author contributions

DF, HP, TS, DR, and KT conceived of the scope of the review and contributed to manuscript preparation. DF conducted the analyses of ToxCast data. TS conducted analyses of NIEHS extramural funding portfolios.

Disclaimer

The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• of special interest
•• of outstanding interest

Conflict of interest statement

Nothing declared.

Acknowledgments

We gratefully acknowledge the contributions of Scott Auerbach, Jerry Heindel, Richard Judson, and Rusty Thomas for reviewing draft versions of the manuscript.

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View full text

Test driving ToxCast: endocrine profiling for 1858 chemicals included in phase II

Highlights

Introduction

Section snippets

Publicly available ToxCast data

Updates to ToxPi prioritization scheme

Updated EDC ToxPi findings

Conclusions

Author contributions

Disclaimer

References and recommended reading

Conflict of interest statement

Acknowledgments

Environ Health

Vom Saal FSCINNRES Pmid: endocrine-disrupting chemicals and public health protection: a statement of principles from The Endocrine Society

Endocrinology

Update on EPA's ToxCast program: providing high throughput decision support tools for chemical risk management

Chem Res Toxicol

Endocrine profiling and prioritization of environmental chemicals using ToxCast data

Environ Health Perspect

GUI: an interactive visualization tool for transparent integration of data from diverse sources of evidence

Bioinformatics

In vitro screening of environmental chemicals for targeted testing prioritization: the ToxCast project

Environ Health Perspect

Aggregating data for computational toxicology applications: the U.S Environmental Protection Agency (EPA) Aggregated Computational Toxicology Resource (ACToR) System

Int J Mol Sci