Sex-specific effects of 6PPD on adult zebrafish: male-selective feeding inhibition, nutrient malabsorption, intestinal epithelial damage, and absence of reproductive toxicity

The tire-derived antioxidant 6PPD (N-(1,3-dimethylbutyl)- N ′-phenyl- p -phenylenediamine), a ubiquitous environmental contaminant, poses significant risks to aquatic ecosystems, yet its sex-specific physiological impacts remain underexplored. This study investigated the effects of environmentally relevant 6PPD concentrations (20 μg/L) on feeding, digestion, absorption, and reproduction in adult zebrafish over 28 days. Results revealed pronounced male-specific impairments: 6PPD-exposed males exhibited reduced feeding speed and maximum food intake, alongside suppressed locomotor responses to food stimuli. Mechanistically, upregulation of intestinal cholecystokinin b ( cckb ) and delayed intestinal content emptying were identified as potential drivers of feeding inhibition in males. Concurrently, 6PPD induced intestinal oxidative stress in males, manifested through decreased activities of catalase (CAT) and superoxide dismutase (SOD), alongside elevated malondialdehyde (MDA) levels, and activated ferroptosis via dysregulation of critical genes ( gpx4a , cybb , slc7a11 , hmox1a , tfr1b and trf2 ). These disruptions correlated with shortened intestinal villi, goblet cell loss, and impaired nutrient absorption, leading to increased fecal output and diminished growth performance in males. In contrast, females displayed no significant structural damage to intestinal epithelium or declines in digestive and absorptive capacities. Notably, 6PPD caused no gonadal histopathological changes, hormonal dysregulation, reduced fertilization and hatching rates, nor transgenerational effects such as developmental abnormalities or locomotor deficits in offspring. This study provides the first evidence of male-selective feeding suppression and intestinal toxicity induced by 6PPD, highlighting sex-dependent vulnerability in aquatic species and underscoring the need for gender-specific risk assessments of tire-derived pollutants.