Development of a screening approach to detect thyroid disrupting chemicals that inhibit the human sodium iodide symporter (NIS)

- Screening assay developed to identify inhibitors of iodide uptake via sodium/iodide symporter (NIS).
- Consistent assay performance demonstrated with reference chemicals and unknown toxicants.
- Guidance provided for data analysis and critical interpretation of concentration response curves.
- Chemical prioritization suggested with use of selectivity scores.
- Approach amenable for screening large chemical libraries for potential thyroid toxicants

The U.S. EPA's Endocrine Disruptor Screening Program aims to use high-throughput assays and computational toxicology models to screen and prioritize chemicals that may disrupt the thyroid signaling pathway. Thyroid hormone biosynthesis requires active iodide uptake mediated by the sodium/iodide symporter (NIS). Monovalent anions, such as the environmental contaminant perchlorate, are competitive inhibitors of NIS, yet limited information exists for more structurally diverse chemicals. A novel cell line expressing human NIS, hNIS-HEK293T-EPA, was used in a radioactive iodide uptake (RAIU) assay to identify inhibitors of NIS-mediated iodide uptake. The RAIU assay was optimized and performance evaluated with 12 reference chemicals comprising known NIS inhibitors and inactive compounds. An additional 39 chemicals including environmental contaminants were evaluated, with 28 inhibiting RAIU over 20% of that observed for solvent controls. Cell viability assays were performed to assess any confounding effects of cytotoxicity. RAIU and cytotoxic responses were used to calculate selectivity scores to group chemicals based on their potential to affect NIS. RAIU IC50 values were also determined for chemicals that displayed concentration-dependent inhibition of RAIU (≥ 50%) without cytotoxicity. Strong assay performance and highly reproducible results support the utilization of this approach to screen large chemical libraries for inhibitors of NIS-mediated iodide uptake.