Thyroid and growth hormone endocrine disruption and mechanisms of homosalate and octisalate using wild-type, thrαa-/-, and dre-miR-499-/- zebrafish embryo/larvae

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Homosalate (HS) and octisalate (OS), which are used in sunscreen for the purpose of blocking ultraviolet rays, are frequently detected in water environment. Although effects on estrogens and androgens have been reported, studies on thyroid and growth hormone endocrine disruption are limited. In the present study, larval mortality was compared in wild-type and two knockout fish (thyroid hormone receptor alpha a knockout (thrαa-/-) and dre-miR-499 knockout (dre-miR-499-/-)) after 96 h of exposure to HS and OS (0, 0.003, 0.03, 0.3, 3, 30 and 300 µg/L). To investigate the mechanisms of thyroid and growth hormone endocrine disruption, we measured the levels of triiodothyronine (T3), thyroxine (T4), thyroid stimulating hormone (TSH), growth hormone (GH), and insulin-like growth factor-1 (IGF-1), and the regulation of representative genes related to the hypothalamus-pituitary-thyroid (HPT) and GH/IGF axis in wild-type zebrafish exposed to target chemicals. The significantly lower larval survival rate of thrαa-/- and dre-miR-499-/- fish exposed to 300 μg/L of HS and OS suggest that thyroid hormone receptors and dre-miR-499 play a crucial role in the toxic effects of HS and OS. The finding of a significant increase in T3 and T4 in zebrafish larvae exposed to HS and OS supports a significant decrease in the crh gene. The reduction of GH and IGF-1 in fish exposed to HS and OS is well supported by the regulation of genes involved in the GH/IGF axis. Our observations suggest that exposure to HS and OS affects not only thyroid hormone receptors and their associated miRNAs, but also the feedback routes of HPT and GH/IGF axes, ultimately leading to growth reduction.