Effects of co-exposure of polyethylene nanoplastics and 6PPD on cardiac function in the early developmental stage of zebrafish.

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is a rubber antioxidant commonly used to manufacture automobile tires. The tire-road wear particles (TRWP) generated during the driving of automobiles, carried by rainwater and surface runoff, will bring the 6PPD and polyethylene microplastics contained in TRWP into the water body, posing a threat to the health and safety of aquatic life. However, the cardiotoxic effects and underlying mechanisms of their combined exposure remain unclear. In this study, zebrafish were exposed to PE-NPs (50 nm in diameter), 6PPD, and their mixtures to investigate cardiac developmental toxicity and molecular mechanisms. The results showed that 6PPD alone caused cardiac phenotypic abnormalities, including shortened SV-BA distance, reduced stroke volume, and decreased heart rate. Co-exposure with PE-NPs further aggravated these toxic phenotypes and significantly disrupted the expression of genes related to myocardial contraction, cardiac development, and heart failure. Mechanistically, PE-NPs enhanced the pericardial accumulation of nanoplastics, triggered more severe ROS overproduction, and induced oxidative stress by upregulating transcription of gpx, cat, and CuZn-SOD while reducing SOD, CAT, and GSH activities. Meanwhile, co-exposure activated the p53-mediated apoptotic pathway, increased the bax/bcl2 ratio, stimulated caspase8 expression, and ultimately aggravated pericardial cell apoptosis in zebrafish larvae. Collectively, PE-NPs exacerbate 6PPD-induced cardiotoxicity in zebrafish early life stages by enhancing pericardial accumulation, amplifying oxidative damage, and promoting apoptosis. This study provides novel mechanistic insights for ecological risk assessment of combined pollution caused by tire-wear-derived 6PPD and nanoplastics in aquatic environments.