Organic and inorganic ultraviolet (UV) filters are a group of substances that either absorb or reflect UV light, preventing it from penetrating the skin (Serpone et al. 2007). The Unites States Food and Drug Administration has classified 22 UV filter compounds, used in sunscreen products, as Generally Recognized As Safe and Effective (GRASE) (category I), those that are not GRASE (category II), and those that do not have sufficient data to support a positive GRASE determination (category III) (US Food and Drug Administration 2019). Avobenzone (also known as butyl methoxydibenzoylmethane) and octinoxate (also known as octyl methoxycinnamate or ethylhexyl methoxycinnamate) are representative components of organic UV filters (Bratkovics et al. 2015), and are classified as category III GRASE (US Food and Drug Administration 2019). As they are often used in sunscreen products, avobenzone and octinoxate are frequently introduced into water (da Silva et al. 2022). Direct release into the marine environment may occur via recreational water activities during swimming and bathing (Labille et al. 2020). Indirect release may occur via wastewater treatment plants (WWTPs) as a result of showering and washing (Poiger et al. 2004).
Avobenzone and octinoxate are frequently detected in the aqueous environment (Ekpeghere et al. 2016; Kameda et al. 2011; Tsui et al. 2014, 2019), sediment (Sun et al. 2021), and biota (Fent et al. 2010; Peng et al. 2017). In surface waters of Hong Kong, Tokyo, New York, Los Angeles, Shantou, Chaozhou, and Bangkok, avobenzone and octinoxate have been detected at 1.37-721 ng/L and 3.84-4043 ng/L, respectively (Tsui et al. 2014, 2019). Octinoxate was detected in streams (21-260 ng/L), heavily polluted rivers (125-1040 ng/L), and sediments (2.0-101 µg/kg) (Kameda et al. 2011). In the sediments of Yalongchangpo river in China, avobenzone was detected at a concentration of 4.13-33.48 ng/g (Sun et al. 2021). In macroinvertebrate and fish samples collected from Swiss rivers, octinoxate occurred up to 337 ng/g in lipids, suggesting bioaccumulation along the food chain (Fent et al. 2010). Octinoxate was the most abundant in three rivers, five sewage treatment plants, and four WWTPs in Korea (Ekpeghere et al. 2016).
Several studies have explored the toxic effects of avobenzone and octinoxate related to oxidative stress (Nataraj et al. 2020) and estrogenic/androgenic effects (Zhou et al. 2019). Avobenzone was confirmed to have thyroid hormone-like activity in GH3-TRE-Luc cells (Klopčič and Dolenc 2017). Production levels of triiodothyronine (T3) and thyroxine (T4), and transcription levels of genes related to type II deiodinase (deio2) were significantly reduced in Japanese medaka exposed to octinoxate (Lee et al. 2019). Treatment with octinoxate caused a decrease of serum thyroid hormone levels in Sprague-Dawley rats (Klammer et al. 2007; Schmutzler et al. 2004). In a recent study, significant decreases of T3 and T4 levels were reported in zebrafish larvae exposed to octinoxate for 120 h (Chu et al. 2021).
T3 and T4 are essential in promoting embryonic development and growth (Walter et al. 2019). Thyroid function depends on synthesis and transport of thyroid hormones, deiodination, iodine uptake, and ability to bind thyroid hormone receptors (Visser 2018). When the levels of T3 and T4 are insufficient, the hypothalamus and pituitary secrete thyrotropin-releasing hormone (TRH) and thyroid stimulating hormone (TSH), respectively (Song et al. 2021). If any of the hormones and enzymes located on the hypothalamus-pituitary-thyroid (HPT) axis are affected by chemical exposure, various effects can be induced at the individual level. Disruption of thyroid hormone homeostasis and developmental toxicity have been studied in fish exposed to several organic UV filters, including benzophenones (BPs; Lee et al. 2018), 4-methylbenzylidene camphor (Quintaneiro et al. 2019), and octinoxate (Chu et al. 2021; Klammer et al. 2007; Lee et al. 2019; Schmutzler et al. 2004). However, information on thyroid disruption by avobenzone is limited, and it is difficult to interpret toxic effects from a holistic point of view using in vitro cell experiments.
In the present study, the effects of avobenzone and octinoxate on zebrafish development and thyroid endocrine system were investigated at the organism-, hormonal-, and genetic-levels. By comparing the mortality rates between wild-type and thyroid hormone receptor alpha a knockout (thrαa−/−) zebrafish larvae, the contribution of both substances to the binding of thyroid hormone receptor was assessed. To fill the knowledge gap on the mechanistic basis of toxicity, the levels of two thyroid hormones and the transcription of HPT axis genes were examined in wild-type zebrafish. The results will provide integrated information elucidating the mechanism of developmental toxicity of avobenzone and octinoxate based on transcriptional and hormonal changes.