Citation:

Olker, J., K. Donnay, J. Haselman, Pat Kosian, Joe Korte, M. Hornung, AND S. Degitz. A tiered approach to evaluate an iodine recycling inhibition adverse outcome pathway (AOP) in amphibians. Society of Toxicology, Baltimore, MD, March 12 - 16, 2017.

Impact/Purpose:

Iodotyrosine deiodinase is the enzyme that catalyzes iodide recycling. This enzyme protects against excretion of critical iodide and promotes accumulation of iodide in the thyroid follicular cells for thyroid hormone synthesis. This iodide recycling is especially critical for low iodine diets and low iodine environments, including most freshwater ecosystems. Evaluating iodotyrosine deiodinase inhibition with in vivo and in vitro assays will establish biological relevance and susceptibility to chemical inhibition, thus expanding coverage of thyroid-specific assays for screening chemicals of concern.

Description:

The enzyme iodotyrosine deiodinase (dehalogenase, IYD) catalyzes iodide recycling and promotes iodide retention in thyroid follicular cells. Loss of function or chemical inhibition of IYD reduces thyroid hormone synthesis, which leads to insufficiency in tissues and subsequent negative developmental consequences. Iodide recycling by IYD is especially critical for low iodine diets and low iodine environments, including most freshwater ecosystems. We developed a putative adverse outcome pathway for IYD inhibition in amphibians and evaluated IYD inhibition with a tiered approach: 1) development of an in vitro IYD enzyme inhibition assay for chemical screening of compounds of interest to the US EPA, 2) ex vivo thyroid culture to establish thyroglobulin iodination as a biomarker of IYD inhibition, and 3) in vivo bioassays to characterize an organismal adverse outcome and test essentiality of IYD activity. An in vitro colorimetric assay was developed to measure activity of recombinant human IYD enzyme in a 96-well format, establishing the feasibility of medium to high throughput screening of chemicals for IYD inhibition. In ex vivo thyroid culture studies, thyroxine (T4), monoiodotyrosine (MIT), and diiodotyrosine (DIT) were quantified in individual thyroid glands and the media using a ultrahigh performance LC-MS/MS. In vivo exposure of developing Xenopus laevis to a suspected IYD inhibitor (3-L-nitro-tyrosine) resulted in markedly delayed metamorphosis and glandular hypertrophy, with dose-response dependent on dietary iodine content (study included two diets with reported iodine content). Compensatory responses were measured via increased sodium/iodide symporter gene expression in the thyroid glands of exposed tadpoles. This multi-pronged approach establishes biological relevance, validates this novel molecular initiating event, links IYD inhibition to the adverse apical outcome (delayed metamorphosis) through measures of biochemical responses, and expands coverage of thyroid-specific assays for screening chemicals of concern to the US EPA.

Record Details: