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Human and Xenopus laevis type 3 iodothyronine deiodinase enzyme cross-species sensitivity to inhibition by ToxCast chemicals
Citation:
Mayasich, S., J. Denny, J. Korte, J. Olker, P. Degoey, J. O'Flanagan, S. Degitz, AND M. Hornung. Human and Xenopus laevis type 3 iodothyronine deiodinase enzyme cross-species sensitivity to inhibition by ToxCast chemicals. SETAC North America, Toronto, ON, CANADA, November 03 - 07, 2019.
Impact/Purpose:
This work helps to address EPA’s need to determine whether chemicals can adversely affect thyroid hormones, which are necessary for amphibian development and metamorphosis. An in vitro assay based on production of amphibian deiodinase enzymes in cultured cell lines has been developed to screen chemicals for their ability to inhibit these enzymes that regulate active thyroid hormone levels in the organism. The results from this effort will provide a basis for making better informed decisions about the predictive potential of screening assays between mammalian and amphibian species.
Description:
Deiodinase enzymes are critical components of the thyroid system that activate or inactivate thyroid hormones when and where needed in vertebrate development, as well as in amphibian metamorphosis. From a screening study of the ToxCast Phase 1, Phase 2 and e1k chemical libraries for inhibitory activity toward human Type 3 iodothyronine deiodinase (DIO 3) enzyme we identified a subset of 357 chemicals to test for amphibian (Xenopus laevis) DIO3 inhibition. Recombinant X. laevis DIO3 enzyme were produced in cell culture and used in 96-well plate screening assays at a single concentration (200 µM). Our goal was to further determine specific chemical inhibitors of DIO3 and cross-species sensitivity. From the initial single concentration screening results with X. laevis DIO3, 80 chemicals were tested in concentration response mode to compare inhibitory activity between the two species. Results showed a general relationship between LogP (octanol:water) and Hill slope. Fourteen of the most potent chemicals with IC50 of 30 µM or less, 10 with LogP 4.5, 33 had steep Hill slopes (much less than -1) for one or both species, with the X. laevis DIO3 more likely to have a higher IC50; for 19 of these chemicals the IC50 for either species was also > 90 µM. Other chemicals with steep Hill slopes for both species but with LogP < 4.5 include 4 PFAS compounds, other surfactants, and chemicals that did not reach complete inhibition at 200 µM. In general, our results indicate that human DIO3 responses to the ToxCast chemicals tested can be predictive of X. laevis DIO3 sensitivity to those chemicals.