You are here:
Screening for chemical inhibition of the iodide recycling enzyme, a novel molecular target for thyroid axis disruption
Citation:
Olker, J., Jonathan Haselman, J. Korte, J. Denny, P. Hartig, M. Cardon, J. O'Flanagan, P. Degoey, M. Hornung, AND S. Degitz. Screening for chemical inhibition of the iodide recycling enzyme, a novel molecular target for thyroid axis disruption. SETAC North America, Toronto, ON, CANADA, November 03 - 07, 2019.
Impact/Purpose:
Iodotyrosine deiodinase (IYD) has an important role in vertebrate thyroid hormone homeostasis through catalyzing iodide recycling and promoting retention of iodide in thyroid follicular cells. This presentation described the development of assays for inhibition of human and amphibian IYD in a 96-well plate format and the application of the human IYD inhibition assay to screen a large set of chemicals. Included are the results from screening over 1,800 chemicals from the ToxCast phase 1_v2, phase 2, and e1k chemical libraries for inhibition of human IYD activity, and the cross-species comparison for ~150 chemicals tested in concentration-response with both human and amphibian IYD. These assays can be used for future screening of large chemical libraries for inhibition of IYD activity. This work supports the U.S. EPA Endocrine Disruptor Screening Program’s need for higher-throughput screening assays to address additional molecular initiating events, beyond those currently in the ToxCast suite of assays, with the potential to disrupt normal thyroid function.
Description:
Environmental contaminants can disrupt thyroid function through a variety of molecular mechanisms, however some putative molecular targets have little known of their toxicological relevance, including susceptibility to chemical perturbation and resulting adverse organismal effects. The iodide recycling enzyme, iodotyrosine deiodinase (IYD), is one conserved putative molecular target that plays an essential role in maintaining adequate levels of free iodide in the thyroid gland for hormone synthesis. Thyroid disruption has recently been demonstrated in a model amphibian (Xenopus laevis) following in vivo IYD inhibition. These effects, along with previously documented effects in mammals, support the biological importance of IYD for proper thyroid function. Here we present development and application of screening assays to assess susceptibility of IYD to chemical perturbation and cross-species concordance between two IYD orthologues. With recombinant human IYD (hIYD) enzyme and Xenopus laevis liver microsomal fractions (xIYD), 96-well plate in vitro assays were developed to screen chemicals for inhibition of mammalian and amphibian IYD enzyme activity. The ToxCast phase 1_v2, phase 2, and e1k chemical libraries were screened using the hIYD assay. Of the over 1,800 unique chemicals tested, less than 200 (~11%) inhibited hIYD activity by 20% or greater at a single, high concentration (target of 200 µM). Further testing of 155 chemicals in concentration-response was conducted to determine IC50s and rank-order potency; this set of chemicals included 83 that produced inhibition of 50% or greater, 22 that produced low to moderate inhibition (20-50%), and 50 non-inhibitors. These same 155 chemicals were tested in concentration-response using the xIYD assay. There was strong cross-species agreement between inhibition of hIYD and xIYD (similar maximum inhibition and potency across most chemicals), with a few notable exceptions. These results greatly expand the number of compounds tested for inhibition of IYD and suggest that, in general, response of IYD activity to potential chemical inhibitors is conserved across these vertebrate species. Combining in vitro screening results, such as presented here, with targeted in vivo testing lends an iterative approach for characterizing the causal linkages between chemical inhibition of a specific molecular target and thyroid-related adverse outcomes.