In Vitro Screening of 149 PFAS Chemicals for Potential Inhibition of the Sodium Iodide Symporter (NIS)#

There is increased concern for environmental chemicals that can target various sites within the hypothalamic-pituitary-thyroid axis to disrupt thyroid synthesis, transport, metabolism and/or function.  One well known thyroid target that has been described in both humans and wildlife is the sodium iodide symporter (NIS) that regulates iodine uptake into the thyroid gland, the first key step of thyroid hormone synthesis. Our laboratory pre-validated developed a radioactive iodide uptake (RAIU) high-throughput (HTP) assay using a stably transduced human NIS cell line (hNIS-HEK293T-EPA) to identify chemicals that have the potential for NIS inhibition.  We previously used this RAIU to test more than 2000 chemicals (US EPA’s ToxCast chemical libraries PI_v2, PII and e1K) and identified a subset of those chemicals that significantly inhibited iodide uptake.  Here, we used this HTP screening assay to evaluate a new test set of 149 unique per- and polyfluoroalkyl substances (PFAS) chemicals (ToxCast PFAS library) for potential activity.  In the current evaluation, the 149 blinded samples were screened using a tiered-approach, first in an initial single-concentration (≤100µM) RAIU assay and subsequent evaluation of the chemicals that produced ≥20% inhibition using multi-concentration (MC) response (0.001µM-100µM) testing in parallel RAIU and cell viability assays. Of this set of chemicals tested in the MC assay, 38 of the PFAS chemicals inhibited iodide uptake more than 20% and 25 more than 50% as compared to the control uptake.  To further prioritize the most potent PFAS NIS inhibitors in this set, chemicals were ranked based on iodide uptake and cytotoxicity and then normalized to perchlorate, the known positive.  We repeated our previous findings that both PFOS and PFHxS were potent NIS inhibitors, but also identified novel PFAS chemicals in our test set that inhibited NIS activity. Although further studies are necessary to confirm these effects in vivo, this initial screening effort identifies NIS as a potential molecular target for thyroid disruption by several persistent and structurally diverse PFAS chemicals.  This abstract does not necessarily represent the views or policies of the U.S. Environmental Protection Agency.

Impact/Purpose

The screening of these 149 ToxCast PFAS chemicals represents the first report of such a large group of structurally diverse PFASs investigated for their potential as a molecular target of NIS.  In addition to the previously observed inhibition of hNIS by PFOS and PFHxS that were repeated in the current screening set, a small set of other unique PFAS were identified as active inhibitors of iodide uptake in the hNIS RAIU with minimal interference by chemical cytotoxicity in the parallel assay.  Therefore, the chemical ranking scores incorporated here will be invaluable for further chemical prioritization in thyroid disrupting evaluations. Several of these more potent and highly ranked inhibitors may warrant additional secondary assay testing to further confirm these initial findings, especially given to the current level of priority/concern for these highly persistent chemicals in the environment. Importantly, as with all in vitro assays, considerations of absorption, distribution, metabolism, excretion (ADME) as well as potential compensatory feedback mechanisms of the thyroid system limit direct extrapolation of in vitro findings to altered thyroid synthesis in vivo. 

Citation

Stoker, T., J. Wang, A. Murr, J. Bailey, AND A. Buckalew. In Vitro Screening of 149 PFAS Chemicals for Potential Inhibition of the Sodium Iodide Symporter (NIS)#. Society of Toxicology, Nashville, TN, March 19 - 23, 2023.