Non-additive effects of norethisterone and levonorgestrel mixtures on lipid metabolism at environmentally relevant...

Norethindrone (NET) and levonorgestrel (LNG) are synthetic progestins frequently detected in aquatic environments, have unclear effects on lipid metabolic homeostasis during the early life stages of aquatic organisms. Although progestins commonly occur as mixtures, their combined impacts remain unclear. In this study, we investigated the individual and combined impacts of NET and LNG at environmentally relevant concentrations (2-200 ng/L) on lipid metabolism in zebrafish larvae. NET and LNG significantly disrupted early development in zebrafish. It also altered lipid profiles, as indicated by elevated triglyceride (TG) levels, reduced total cholesterol (TC), as well as alterations in key metabolic enzymes (FASN, LPL) and lipid-regulatory genes (pparγ, fasn, lpl, pparα). Co-exposure with LNG resulted in non-additive responses across multiple endpoints. Antagonistic interactions were predominant at medium and high concentrations, while occasional synergism was observed at low doses. These complex patterns were further supported by Bliss independence model analysis. Notably, combined exposure suppressed both lipid synthesis and degradation pathways more strongly than individual treatments, leading to lipid accumulation and altered energy regulation. This study advanced understanding of the ecological risks caused by progestins in aquatic environments and highlighted the necessity of mixture-based risk assessment of endocrine-disrupting compounds.