Thyroid endocrine disruption in zebrafish larvae following exposure to hexaconazole and tebuconazole

Abstract

The widely used triazole fungicides have the potential to disrupt endocrine system, but little is known of such effects or underlying mechanisms of hexaconazole (HEX) and tebuconazole (TEB) in fish. In the present study, zebrafish (Danio rerio) embryos were exposed to various concentrations of HEX (0.625, 1.25 and 2.5 mg/L) and TEB (1, 2 and 4 mg/L) from fertilization to 120 h post-fertilization (hpf). The whole body content of thyroid hormone and transcription of genes in the hypothalamic-pituitary-thyroid (HPT) axis were analyzed. The results showed that thyroxine (T4) levels were significantly decreased, while triiodothyronine (T3) concentrations were significantly increased after exposure to HEX and TEB, indicating thyroid endocrine disruption. Exposure to HEX significantly induced the transcription of all the measured genes (i.e., corticotrophin-releasing hormone (CRH), thyroid-stimulating hormone (TSHβ), sodium/iodide symporter (NIS), transthyretin (TTR), uridine diphosphate glucuronosyltransferase (UGT1ab), thyronine deiodinase (Dio1 and Dio2), thyroid hormone receptors (TRα and TRβ) in the HPT axis, but did not affect the transcription of thyroglobulin (TG). However, TEB exposure resulted in the upregulation of all the measured genes, excepting that TG, Dio1and TRα had not changed significantly. The overall results indicated that exposure to HEX and TEB could alter thyroid hormone levels as well as gene transcription in the HPT axis in zebrafish larvae.

Thyroid disruption in zebrafish (Danio rerio) larvae: Different molecular response patterns lead to impaired eye development and visual functions

“…Thyroiddisrupting effects have been reported for, e.g. polychlorinated biphenyls, polyaromatic hydrocarbons, polybrominated diphenyl ethers, organochlorine pesticides as well as different metals (Brown et al, 2004;Yu et al, 2013). Those environmental chemicals appear to interfere with the thyroid system of vertebrates by two principal molecular mechanisms: a) Disrupting the thyroid hormone homeostasis, which includes effects on enzymes responsible for iodine uptake and thyroid hormone synthesis, and transport in the blood, on cellular uptake degradation and excretion = thyroid gland function disruptors (Thienpont et al, 2011).…”

Thyroid endocrine system disruption by pentachlorophenol: An in vitro and in vivo assay

“…Recently, an in vivo model for testing endocrine disruption of THs was developed using zebrafish larvae (Yu et al, 2010;Schmidt and Braunbeck, 2011). This assay indicated that the zebrafish HPT axis can be used to determine thyroid endocrine system disruption by TH endocrine disruptors, and also, to some extent, to indicate potential mechanisms of action, e.g., polybrominated diphenyl ethers (PBDEs) (Yu et al, 2010;Chan and Chan, 2012), herbicide ioxynil (Campinho and Power, 2013), fungicides hexaconazole and tebuconazole (Yu et al, 2013), and triclosan (Pinto et al, 2013). However, the disruption of the thyroid endocrine system by PCP and the environmental risk to fish remains unknown.…”

Toxicological effects of clofibric acid and diclofenac on plasma thyroid hormones of an Indian major carp, Cirrhinus mrigala during short and long-term exposures

“…A lower level of T 4 may be due to an increased activity of UDP-transferase which responsible for glucuronidation and clearance of T 4 (Zhou et al, 2001;Hallgren and Darnerud, 2002). Yu et al (2013) reported that a significant decrease in T 4 level in D. rerio exposed to the fungicides hexaconazole and tebuconazole indicate thyroid endocrine disruption. A significant decrease of T 4 levels was observed in zebrafish larvae with decabromodiphenyl ether (BDE-209) exposure whereas T 3 levels were significantly increased indicating that polybrominated diphenyl ethers (PBDE) induce thyroid endocrine disruption by disturbing T 4 levels, but the changes in T 3 levels may depend on exposure time, species or concentrations.…”