6PPD and its metabolite 6PPDQ induce different developmental toxicities and phenotypes in embryonic zebrafish

The automobile tire antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone metabolite 6PPDQ have recently received much attention for their acute aquatic toxicity. The present study investigated the mechanistic developmental toxicity of 6PPD and 6PPDQ in embryonic zebrafish. Neither compound induced significant mortality but significantly decreased spontaneous embryo movement and heart rate. Both compounds induced malformations with different phenotypes; the 6PPD-exposed larvae manifested a myopia-like phenotype with a convex eyeball and fusion vessels, while the 6PPDQ-exposed embryonic zebrafish manifested enlarged intestine and blood-coagulated gut, activated neutrophils, and overexpressed enteric neurons. mRNA-Seq and quantitative real-time PCR assays showed that 6PPD- and 6PPDQ-induced distinct differential gene expression aligned with their toxic phenotype. 6PPD activated the retinoic acid metabolic gene cyp26a, but 6PPDQ activated adaptive cellular response to xenobiotics gene cyp1a. 6PPD suppressed the gene expression of the eye involved in retinoic acid metabolism, phototransduction, photoreceptor function and visual perception. In contrast, 6PPDQ perturbed genes involved in inward rectifier K⁺ and voltage-gated ion channels activities, K⁺ import across the plasma membrane, iron ion binding, and intestinal immune network for IgA production. The current study advances the present understanding the reason of why many fish species are so adversely impacted by 6PPD and 6PPDQ.