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Development of a Screening Approach to Detect Thyroid Disrupting Chemicals that Inhibit the Human Sodium/Iodide Symporter (NIS)
Citation:
Hallinger, D., A. Murr, A. Buckalew, Steve Simmons, T. Stoker, AND S. Laws. Development of a Screening Approach to Detect Thyroid Disrupting Chemicals that Inhibit the Human Sodium/Iodide Symporter (NIS). TOXICOLOGY IN VITRO. Elsevier Science Ltd, New York, NY, 40:66-78, (2017).
Impact/Purpose:
This work was conducted to support the Agency’s Endocrine Disruptor Screening Program and its development of screening approaches to identify potential thyroid toxicants. Research was conducted under CSS 16.01 Task 2.1. The information will be used by OSCP, OPP, OPPT and OW as they implement EDSP and will contribute to the ability to expand our coverage of known molecular targets of chemical-induced thyroid disruption. In addition, this assay will be used to screen the ToxCast libraries and also will support the development of international test guidelines.
Description:
Thyroid hormone synthesis requires active iodide uptake mediated by the sodium/iodide symporter (NIS). Monovalent anions, such as the environmental contaminant perchlorate, have been well characterized as competitive inhibitors of NIS, yet limited information exists for more structurally diverse chemicals. Here we report the implementation of a novel cell line that stably expresses functional human NIS, hNIS-HEK293T-EPA, in an approach to identify potential inhibitors of NIS-mediated iodide uptake. A radioactive iodide uptake (RAIU) assay was optimized and its performance demonstrated with a 12 chemical reference set including known NIS inhibitors and recognized inactive chemicals. A 39 chemical test set including environmental toxicants was then evaluated using the assay, which identified 28 chemicals that inhibited RAIU. Cell viability assays and Sandell-Kolthoff reactions were performed to assess any confounding effects of cytotoxicity or contaminant iodide on RAIU response. While none of the chemicals contained appreciable iodide content, RAIU inhibition for 17 chemicals was confounded by some level 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 determined for chemicals that displayed concentration dependent inhibition of RAIU (≥ 50%) without causing cytotoxicity. Two dithiocarbamates, mancozeb and metiram, and an antibacterial, triclocarban, exhibited inhibitory RAIU activity (IC50: 39.5, 50.7, and 5.7uM) that might warrant further investigation. 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.
