Niacin Ameliorates EHDPHP-Induced Oxidative Stress and Mitochondrial Dysfunction in H9C2 Cells

2-Ethylhexyl diphenyl phosphate (EHDPHP) is a widely used organophosphorus flame retardant frequently detected in environmental matrices and poses potential health risks. However, its cardiotoxic effects on mammalian cardiomyocytes and the underlying molecular mechanisms remain largely unclear. Niacin (NIA), an essential water-soluble vitamin, exhibits potent antioxidant, anti-inflammatory, and mitochondrial-protective activities. In this study, we investigated EHDPHP-induced toxicity in H9C2 cardiomyocytes and the protective effects of NIA. Cells were exposed to 100 μM EHDPHP alone or in combination with 400, 600, and 800 μM NIA. EHDPHP exposure significantly reduced cell viability, disrupted the balance between pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, and IL-18) and the anti-inflammatory cytokine IL-10, and induced oxidative stress, as evidenced by elevated ROS, MitoSOX, and MDA levels alongside decreased activities of antioxidant enzymes (SOD, GSH, GSH-Px, and CAT). Additionally, EHDPHP disturbed mitochondrial dynamics by promoting fission and inhibiting fusion, impaired mitochondrial biogenesis via downregulation of AMPK and PGC-1α, triggered excessive mitophagy through PINK1, PRKN, and LC3 upregulation, and activated pyroptosis via GSDMD, NLRP3, and Caspase-1. Bioinformatics analyses confirmed the interconnected regulatory network among mitochondrial dynamics, mitophagy, pyroptosis, and inflammatory signaling in EHDPHP-induced cardiomyocyte injury. Notably, NIA intervention dose-dependently mitigated these detrimental effects, restoring cell viability, alleviating inflammation and oxidative stress, rebalancing mitochondrial fusion and fission, rescuing biogenesis, normalizing mitophagy, and inhibiting pyroptosis. These findings reveal a pathological cascade through which EHDPHP induces cardiomyocyte injury and demonstrate that NIA confers cardioprotection by targeting multiple pathological pathways. This study provides novel mechanistic insights into EHDPHP-induced cardiotoxicity and highlights NIA as a promising nutritional intervention for reducing cardiovascular risks associated with environmental pollutants.