High-Throughput Screening of ToxCast PFAS Chemical Library for Potential Inhibitors of the Human Sodium Iodide Symporter

Abstract

Over the past decade, there has been increased concern for environmental chemicals that can target various sites within the hypothalamic–pituitary–thyroid axis to potentially disrupt thyroid synthesis, transport, metabolism, and/or function. One well-known thyroid target in both humans and wildlife is the sodium iodide symporter (NIS) that regulates iodide uptake into the thyroid gland, the first step of thyroid hormone synthesis. Our laboratory previously developed and validated a radioactive iodide uptake (RAIU) high-throughput assay in a stably transduced human NIS cell line (hNIS-HEK293T-EPA) to identify chemicals with potential for NIS inhibition. So far, we have tested over 2000 chemicals (US EPA’s ToxCast chemical libraries PI_v2, PII, and e1K) and discovered a subset of chemicals that significantly inhibit iodide uptake in the hNIS assay. Here, we utilized this screening assay to test a set of 149 unique per- and polyfluoroalkyl substances (PFAS) (ToxCast PFAS library) for potential NIS inhibition. For this evaluation, the 149 blinded samples were screened in a tiered approach, first in an initial single-concentration (≤100 μM) RAIU assay and subsequent evaluation of the chemicals that produced ≥20% inhibition using multiconcentration (MC) response (0.001–100 μM) testing in parallel RAIU and cell viability assays. Of this set, 38 of the PFAS chemicals inhibited iodide uptake ≥20% in the MC testing with 25 displaying inhibition ≥50%. To prioritize the most potent PFAS NIS inhibitors in this set, chemicals were ranked based on outcomes of both iodide uptake and cytotoxicity and normalized to perchlorate, a known positive control. Consistent with previous findings, PFOS and PFHxS were again found to be potent NIS inhibitors, yet significant inhibition was also observed for several other screened PFAS chemicals. Although further studies are clearly warranted, this initial screening effort identifies NIS as a molecular target for potential thyroid disruption by this persistent and structurally diverse class of chemicals.

Baseline Toxicity Model to Identify the Specific and Nonspecific Effects of Per- and Polyfluoroalkyl Substances in Cell-Based Bioassays

“…Those PFAS with high values of TR and SR on defined targets may provide some hints on initial molecular events and key events, which will facilitate the in vitro to in vivo extrapolation to adverse outcomes and thus promote the development of AOP related to PFAS. However, current results of cell-based HTS from EPA suggest that a majority of 160 PFAS were inactive or equivocal. − …”

“…A NAM battery for developmental neurotoxicity was developed to evaluate the effects of 160 PFAS on neural network formation and function . Radioactive iodide uptake high-throughput assay was used to test 149 PFAS for potential thyroid disruption . However, data evaluation and ranking scores were done independently with diverse conclusions, which makes it difficult to determine which PFAS are of the highest concern and which in vitro end points are relevant for human health.…”

“…Tox21 and ToxCast are programs of multiple federal agencies, including the U.S. Environmental Protection Agency (EPA) and the National Toxicology Program, aiming at efficiently identifying potential AOPs for specified chemicals at molecular, cellular, and organ levels through HTS to facilitate risk assessments of chemicals. Currently, in vitro effect data for 160 PFAS from the EPA have been published. − Transactivation assays encompassing 81 diverse transcription factors were screened with 142 PFAS to describe the activation of nuclear receptor-mediated signaling pathways . 148 biomarkers relevant to the immune system were measured with 147 PFAS to inform mechanisms of immunotoxicity .…”