Perturbation of the Reproductive Endocrine System by Halogenated Nitrophenols via Thyroid Receptor Signaling: Implications for Developmental, Fertility, and Intergenerational Risks in Japanese Medaka

Abstract Disinfection byproducts (DBPs) are ubiquitously formed in disinfected drinking water and wastewater effluents, with emerging aromatic DBPs constituting a large fraction and generally exhibiting higher toxicity than regulated species. However, their ecological risks remain poorly understood. Here, we focused on two typical halogenated nitrophenols, 2,6-dibromo-4-nitrophenol (2,6-DB-4-NP) and 2,6-dichloro-4-nitrophenol (2,6-DC-4-NP), and systematically evaluated their developmental and reproductive toxicity in Japanese medaka at environmentally relevant ng/L concentrations. Early-life exposure significantly delayed hatching and increased larval mortality, while a 110-day chronic exposure inhibited further gonadal development. These effects were linked to disruption of thyroid hormone homeostasis, reflected by an altered triiodothyronine/thyroxine (T3/T4) ratio and dysfunction of the hypothalamic–pituitary–thyroid (HPT) axis, which subsequently led to sex-specific dysregulation of the hypothalamic–pituitary–gonadal (HPG) axis. Males displayed elevated estrogen levels, enhanced estrogen receptor signaling, and hepatic vitellogenin synthesis, whereas females showed the opposite pattern, indicating a feminization risk and diminished estrogenic responses, respectively. Furthermore, parental exposure caused developmental impairments in the offspring, suggesting potential intergenerational toxicity. In silico docking further revealed that 14 other aromatic DBPs exhibited high-affinity binding to thyroid hormone receptors (TRα and TRβ), similar to the two halogenated nitrophenols, supporting their potential to disrupt TR-mediated signaling. Collectively, this study adds to the limited in vivo evidence of thyroid and reproductive endocrine disruption by halogenated nitrophenols under environmentally realistic conditions, underscoring their ecological significance and implicating TR signaling as a mechanistic node for future study.