In Vitro Screening of 149 PFAS Chemicals for Inhibition of the Human Sodium/Iodide Symporter (hNIS)

  The U.S. EPA's Office of Research and Development (ORD) is currently evaluating HTS assays to identify chemicals that disrupt thyroid hormone (TH) homeostasis. One focus is the human sodium/iodide symporter (hNIS), as this transmembrane glycoprotein mediates iodine uptake into the thyroid gland to initiate TH biosynthesis. Radioactive iodide uptake (RAIU) and cell viability assays were previously measured in parallel using the (hNIS-HEK293T-EPA cell line to evaluate over 2000 chemicals and well-characterized NIS inhibitors. Here, we used this hNIS high-throughput screening assay to examine 149 unique per- and polyfluoroalkyl substances (PFAS) compounds (ToxCast PFAS library) for potential inhibition.  Chemicals were screened in a stratified approach, first in the single concentration RAIU assay (≤100 µM) and then in multi-concentration (MC) screening (0.001 µM-100 µM) parallel RAIU and cell viability assays for chemicals that inhibited ≥20%. 38 of the PFAS chemicals tested in this MC assay inhibited iodine uptake by more than 20%, with 25 of them exhibiting more than 50% inhibition. To prioritize the most potent NIS inhibitors, these active test chemicals were ranked and prioritized based on the potency of iodine uptake inhibition and cytotoxicity, then normalized to perchlorate, a well-known environmental NIS inhibitor. PFOS and PFHxS were found to be potent NIS inhibitors, which is consistent with our previous findings, as well as many of the other tested PFAS chemicals. Although further studies are clearly needed, this initial screening effort identifies NIS as a molecular target with potential thyroid disruption by this persistent and structurally diverse class of chemicals. This abstract does not reflect U.S. EPA policy  

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.

Citation

Bailey, J., A. Buckalew, A. Murr, J. Wang, AND T. Stoker. In Vitro Screening of 149 PFAS Chemicals for Inhibition of the Human Sodium/Iodide Symporter (hNIS). EDC-NC, Raleigh, NC, September 30, 2022.