The differential cardiotoxicity effects and molecular mechanisms induced by two emerging organophosphorus flame retardants (EHDPP and DPHP) with similar parent structures in zebrafish

Herein, we systematically compared the cardiotoxicity effects of two structurally similar organophosphate flame retardants (OPFRs; EHDPP and its major metabolite DPHP) using zebrafishi as a model organism. Predictive toxicity data from two aquatic species, together with empirical LC₅₀ values obtained in zebrafish larvae, consistently demonstrated that 2-ethylhexyl diphenyl phosphate (EHDPP) is substantially more toxic than diphenyl phosphate (DPHP). At LC₅₀-scaled concentrations, both contaminants induced the reduced heart rate, pericardial edema, and cardiac malformations, in concomitant with enhanced activities of creatine kinase (CK) and lactate dehydrogenase (LDH) and downregulated cardiac transcription factors ( gata4 and Nkx2.5 ) and upregulated structural gene cmlc2 . Even at low concentrations, EHDPP triggered apoptosis and elevated reactive oxygen species (ROS) levels in cardiac and pericardial tisues, along with prominent lipid accumulation in the heart, vasculature, and intersegmental arteries. It activated multiple pathways including MAPK, calcium signaling, and excitation-contraction coupling, while DPHP primarily affected cardiac function via calcium pump regulation and neuro-signaling pathways. Disease ontology (DO) enrichment analysis revealed that both EHDPP and DPHP associated target genes were most significantly enriched in cardiovascular-related diseases. Pharmacological rescue experiments using pathway-specific agents (Y-27632 and isoproterenol) further validated the functional roles of key signaling pathways in mediating the observed cardiotoxicity. Albeit differing in only one alkyl chain, EHDPP and DPHP displayed significantly differential cardiotoxicity and molecular mechanisms in zebrafish. These observations are conducive to illustrating OPFRs' structure-activity relationship and their health risk. • EHDPP shows stronger developmental and lethal toxicity than DPHP with the same OPR core. • Both induce linear cardiac deformities and cardiomyocyte apoptosis via oxidative stress. • Heart rhythm/function are disrupted by inhibiting key transcription factors. • RNA-seq and GSEA reveal different toxic targets and signaling pathways. • Molecular docking, DO analysis, and RT-qPCR confirm the cardiotoxic mechanisms.