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High-Throughput Screening and Quantitative Chemical Ranking for Sodium Iodide Symporter Inhibitors in ToxCast Phase 1 Chemical Library
Citation:
Wang, J., D. Hallinger, A. Murr, A. Buckalew, Steve Simmons, S. Laws, AND T. Stoker. High-Throughput Screening and Quantitative Chemical Ranking for Sodium Iodide Symporter Inhibitors in ToxCast Phase 1 Chemical Library. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 52(9):5417-5426, (2018).
Impact/Purpose:
This work supports the US EPA Endocrine Disruptor Screening Program’s need for higher-throughput screening assays to address additional molecular initiating events with the potential to disrupt normal thyroid hormone signaling. The research addresses an urgent need and request from the Office of Coordination and Science Policy to develop and implement the use of in vitro assays to screen environmental chemicals for potential thyroid toxicants. Using a novel screening approach, chemicals within the ToxCast Phase 1_v2 chemical library have been screened for the potential to inhibit thyroid hormone synthesis by disrupting the sodium iodide symporter (NIS) -mediated uptake of iodide. The NIS is the critical first step in the synthesis of thyroid hormones in the thyroid gland. The screening and ranking of 293 chemicals in this study represents the first large scale effort to investigate NIS as a molecular target of environmental chemicals with a broad spectrum of structural and toxicological properties. These data can inform the prioritization of chemicals for secondary testing and support the needs of the EDSP.
Description:
The U.S. EPA’s Endocrine Disruptor Screening Program (EDSP) and Office of Research and Development (ORD) are currently developing high throughput assays to screen chemicals that may alter the thyroid hormone pathway. One potential target in this pathway is the sodium iodide symporter (NIS), a transmembrane glycoprotein that mediates iodide uptake into the thyroid as the initial step in thyroid hormone synthesis. Our laboratory recently validated a screening approach to identify potential NIS inhibitors using a novel cell line hNIS-HEK293T-EPA. Here, we screened the ToxCast phase 1_v2 chemical library, representing 293 unique environmental chemicals, in the newly developed radioactive iodide uptake (RAIU) assay. First, 310 blinded samples were screened in single-concentration format at 100µM. Using 20% inhibition as the activity threshold, 169 samples reached criteria and were further tested for concentration-response (0.001 µM - 100 µM) with parallel RAIU and cell viability assays. To facilitate prioritization of potential thyroid toxicants, a unique ranking system that incorporates cytotoxicity responses was developed to sort the RAIU inhibition potency of the chemicals. Chemicals were ranked with a score that is relative to the well-known NSI inhibitor sodium perchlorate. The top ranked 14 chemicals feature diverse toxicological properties that were discussed in detail and their varying responses in the screening warrant further testing to characterize the nature of the inhibition. This study represents the first large-scale screening of environmental chemicals for NIS inhibitors, and supports the U.S. EPA’s EDSP21 and OECD in their efforts to develop high throughput screening approaches that expand the coverage of molecular targets of thyroid disruption.
