BPA exposure activated estrogen receptor α (ER-α) and ROS to induce pyroptosis in cochlear hair cells

Increasing evidence links hearing loss to environmental pollutant exposure, with cochlear hair cell being a key target. Bisphenol A (BPA) has been suggested to cause ototoxicity, but its mechanism is not fully understood. In this study, cell viability was assessed by varying concentrations of BPA (0-150 μM) exposed to mouse cochlear hair cells (HEI-OC1), followed by RNA transcriptome sequencing to identify the possible molecular mechanisms. Results showed that BPA above 50 μM reduced cell viability in a dose-dependent manner. RNA sequencing identified 1764 differentially-expressed genes, comprised of 1152 up-regulated and 612 down-regulated genes. Notably, estrogen receptor-related genes were enriched, with a marked up-regulation of estrogen receptor α (ER-α) at both mRNA and protein levels. GO and KEGG analyses revealed the main pathway involvement for oxidative damage, lipid metabolism, protein phosphorylation, and DNA damage. STRING analyses constructed gene networks to identify functionally interacting genes correlated with pyroptotic pathways. BPA exposure induced pyroptosis-like morphological changes characterized by cell swelling, rounding, and membrane blebbing. Increased intracellular ROS and mitochondrial superoxide (MitoSOX) levels, reduced mitochondrial membrane potential (ΔΨm), and elevated intracellular calcium (Ca²⁺) levels were observed. Flow cytometry showed an increased proportion of apoptotic and necrotic cells. Pyroptosis-related genes, including caspase-3, caspase-8, caspase-9, and GSDME, were down-regulated, while IL-1β and IL-18 were up-regulated. Protein expressions of caspase-3, caspase-8, caspase-9, and GSDME were decreased, while cleaved caspase-3 and N-terminal GSDME (GSDME-NT) were increased. Furthermore, ROS inhibitor N-acetylcysteine (NAC) and the ER-α antagonist fulvestrant alleviated BPA-induced cell death, and suppressed the protein expressions of the pyroptotic pathway. These results demonstrate that BPA exposure induces ototoxicity possibly through activating ER-α and triggering mitochondrial ROS that contributes to pyroptosis.