title: Thyroid tumor formation in the male mouse induced by fluopyram is mediated by activation of hepatic CAR/PXR nuclear receptors.
abstract: Fluopyram, a broad spectrum fungicide, caused an increased incidence of thyroid follicular cell (TFC) adenomas in males at the highest dose evaluated (750ppm equating to 105mg/kg/day) in the mouse oncogenicity study. A series of short-term mechanistic studies were conducted in the male mouse to characterize the mode of action (MOA) for the thyroid tumor formation and to determine if No Observed Effect Levels (NOELs) exist for each key event identified. The proposed MOA consists of an initial effect on the liver by activating the constitutive androstane (Car) and pregnane X (Pxr) nuclear receptors causing increased elimination of thyroid hormones followed by an increased secretion of thyroid stimulating hormone (TSH). This change in TSH secretion results in an increase of TFC proliferation which leads to hyperplasia and eventually adenomas after chronic exposure. Car/Pxr nuclear receptors were shown to be activated as indicated by increased activity of specific Phase I enzymes (PROD and BROD, respectively). Furthermore, evidence of increased T4 metabolism was provided by the induction of phase II enzymes known to preferentially use T4 as a substrate. Additional support for the proposed MOA was provided by demonstrating increased Tsh β transcripts in the pituitary gland. Finally, increased TFC proliferation was observed after 28days of treatment. In these dose-response studies, clear NOELs were established for phase 2 liver enzyme activities, TSH changes and TFC proliferation. Furthermore, compelling evidence for Car/Pxr activation being the molecular initiating event for these thyroid tumors was provided by the absence of the sequential key events responsible for the TCF tumors in Car/Pxr KO mice when exposed to fluopyram. In conclusion, fluopyram thyroid toxicity is mediated by activation of hepatic Car/Pxr receptors and shows a threshold dependent MOA.
authors.affiliation: Bayer SAS, Bayer CropScience, 355 rue Dostoïevski, CS 90153, 06906 Sophia Antipolis, France. Electronic address: david.rouquie@bayer.com.
authors.forename: D
authors.initials: D
authors.lastname: Rouquié
authors.affiliation: Bayer SAS, Bayer CropScience, 355 rue Dostoïevski, CS 90153, 06906 Sophia Antipolis, France.
authors.forename: H
authors.initials: H
authors.lastname: Tinwell
authors.affiliation: Bayer SAS, Bayer CropScience, 355 rue Dostoïevski, CS 90153, 06906 Sophia Antipolis, France.
authors.forename: O
authors.initials: O
authors.lastname: Blanck
authors.affiliation: Bayer SAS, Bayer CropScience, 355 rue Dostoïevski, CS 90153, 06906 Sophia Antipolis, France.
authors.forename: F
authors.initials: F
authors.lastname: Schorsch
authors.affiliation: Bayer SAS, Bayer CropScience, 355 rue Dostoïevski, CS 90153, 06906 Sophia Antipolis, France.
authors.forename: D
authors.initials: D
authors.lastname: Geter
authors.affiliation: Bayer SAS, Bayer CropScience, 355 rue Dostoïevski, CS 90153, 06906 Sophia Antipolis, France.
authors.forename: S
authors.initials: S
authors.lastname: Wason
authors.affiliation: Bayer SAS, Bayer CropScience, 355 rue Dostoïevski, CS 90153, 06906 Sophia Antipolis, France.
authors.forename: R
authors.initials: R
authors.lastname: Bars
mesh_terms: D000818:Animals; D001549:Benzamides / Q000633:toxicity*; D049109:Cell Proliferation / Q000187:drug effects; D000090702:Constitutive Androstane Receptor; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D005659:Fungicides, Industrial / Q000633:toxicity*; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D019055:No-Observed-Adverse-Effect Level; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism; D000077297:Pregnane X Receptor; D011725:Pyridines / Q000633:toxicity*; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D011987:Receptors, Steroid / Q000235:genetics / Q000378:metabolism*; D013961:Thyroid Gland / Q000166:cytology / Q000187:drug effects; D013964:Thyroid Neoplasms / Q000378:metabolism*; D013972:Thyrotropin / Q000097:blood / Q000235:genetics / Q000378:metabolism; D013974:Thyroxine / Q000097:blood
chemical_list: D001549:Benzamides; D000090702:Constitutive Androstane Receptor; D005659:Fungicides, Industrial; D000077297:Pregnane X Receptor; D011725:Pyridines; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; D013972:Thyrotropin; D003577:Cytochrome P-450 Enzyme System; C572868:N-(2-(3-chloro-5-(trifluoromethyl)-2-pyridyl)ethyl)-alpha,alpha,alpha-trifluoro-o-toluamide; D013974:Thyroxine
keywords: Car/Pxr nuclear receptors; Mode of action; Mouse thyroid carcinogen; Threshold carcinogen; Thyroid follicular cell proliferation