Citation:

Stoker, T., J. Wang, D. Hallinger, A. Murr, A. Buckalew, Steve Simmons, AND S. Laws. High-throughput screening (HTPS) of ToxCast Phase II chemical library for sodium iodide symporter (NIS) Inhibitors. Society of Toxicology, San Antonio, Texas, March 11 - 15, 2018.

Impact/Purpose:

High-throughput screening results for sodium iodide symporter inhibition using ToxCast Phase II chemical library.

Description:

In support of the Endocrine Disruptor Screening Program (EDSP), the U.S. EPA’s Office of Research and Development (ORD) is currently developing HTPS approaches to identify chemicals that may alter target sites in the thyroid hormone pathway. One target site is the sodium iodide symporter (NIS), a transmembrane glycoprotein that mediates iodide uptake into the thyroid gland as the initial step of thyroid hormone biosynthesis. Previously, we validated a HTPS approach using a novel cell line (hNIS-HEK293T-EPA) in parallel radioactive iodide uptake (RAIU) and cell viability assays to identify potential environmental NIS inhibitors. These assays were successfully used to screen a set of 50 test chemicals as well as 293 ToxCast Phase I chemicals. Here, we report the continued screening effort to identify potential NIS inhibitors in the ToxCast Phase II chemical library (777 blinded samples). Initially, samples were screened in single-concentration format with a two-hour exposure at 100µM in the RAIU assay. Using 20% inhibition of iodide uptake as the hit-call threshold, a total of 235 samples were further tested in multi-concentration (0.001 - 100µM) format in both RAIU and cell viability assays, including known NIS inhibitors and negative chemicals across all plates. Of these, 168 samples exhibited significant RAIU inhibition and 67 samples did not. The 168 positive samples were then prioritized as potential thyroid toxicants using a unique ranking system to sort the RAIU inhibition potency by applying a score relative to the well-known NIS inhibitor perchlorate and cytotoxicity. Some of the highest ranked chemicals, such as PFOS, nitrofen, tributyltin chloride, and triclocarban have been previously reported to be thyroid disruptors. A secondary in vitro rat thyroid follicular cell line (FRTL5) will be utilized to confirm and further characterize the top-ranked chemicals across ToxCast chemical libraries. This abstract does not reflect U.S.EPA policy.

Record Details: