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Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases
Citation:
Olker, J., Joe Korte, J. Denny, P. Hartig, M. Cardon, C. Knutsen, P. Kent, J. Christensen, S. Degitz, AND M. Hornung. Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases. TOXICOLOGICAL SCIENCES. Society of Toxicology, RESTON, VA, 168(2):430-442, (2019). https://doi.org/10.1093/toxsci/kfy302
Impact/Purpose:
Deiodinase enzymes perform critical roles in vertebrates by maintaining the balance between having sufficient active thyroid hormone when and where it is needed, and the inactivation of hormone when signaling needs to be reduced. Development of a 96-well plate assay to assess chemical inhibition of deiodinase types 1, 2, and 3 activity in vitro and screening of large chemical libraries is presented in this manuscript. 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.
Description:
Deiodinase enzymes play an essential role in converting thyroid hormones between active and inactive forms by deiodinating the pro-hormone thyroxine (T4) to the active hormone triiodothyronine (T3) and modifying T4 and T3 to inactive forms. Chemical inhibition of deiodinase activity has been identified as an important endpoint to include in screening chemicals for thyroid hormone disruption. To address the lack of data regarding chemicals that inhibit the deiodinase enzymes, we developed robust in vitro assays for inhibition of human deiodinase types 1, 2, and 3 enzyme activity and screened over 1,800 unique chemicals from the EPA’s ToxCast phase 1_v2, phase 2, and e1k libraries. Initial testing at a single concentration identified 411 putative deiodinase inhibitors that produced inhibition of 20% or greater in at least one of the three deiodinase assays, including chemicals that have not previously been shown to inhibit deiodinases. Of these, 228 chemicals produced enzyme inhibition of 50% or greater; these chemicals were further tested in concentration-response mode to determine relative potency. Over 90% of the tested chemicals had similar results across the three deiodinases; however, possible deiodinase-specific inhibition was produced by 81 chemicals. This set of deiodinase inhibition assays is significant contribution towards expanding the limited number of in vitro assays used to identify chemicals with the potential to interfere with thyroid hormone homeostasis. Additionally, these results set the groundwork for development and evaluation of structure-activity relationships for deiodinase inhibition, and inform targeted selection of chemicals for further testing to identify adverse outcomes of deiodinase inhibition.
URLs/Downloads:
DOI: Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases