Aryl Hydrocarbon Receptor (Ahr) Pathway Drives TBBPA Induced Cartilage Development Defects.

Tetrabromobisphenol A (TBBPA), is one the most widely produced brominated flame retardant, detected in human matrices including cord plasma, raising concern over its impact on embryonic development. Our previous work demonstrated that TBBPA disrupts craniofacial cartilage development in zebrafish; however, inhibition of bone morphogenetic protein (BMP) signaling did not rescue these defects, suggesting the involvement of alternative molecular mechanisms. Transcriptomic profiling revealed significant upregulation of aryl hydrocarbon receptor (Ahr) signaling genes, including cyp1a and cyp1c, as well as reactive oxygen species (ROS) responsive genes, such as nfe and gstp. Consistent with these findings, KEGG pathway enrichment analysis identified significant enrichment of pathways involved in xenobiotic metabolism, cytochrome P450-mediated metabolism, and molecular docking predicted stronger binding of TBBPA to Ahr2 than the Ahr2 agonist TCDD. Then we investigated the role of Ahr signaling and ROS in TBBPA-induced craniofacial cartilage defects using microinjection of Ahr2 translation-blocking morpholino and the antioxidant N-acetylcysteine (NAC), respectively. Immunohistochemistry confirmed concentration-dependent induction of Cyp1a and ROS at environmentally relevant concentrations (0.05;0.005 uM). Ahr2 knockdown using a translation-blocking morpholino rescued TBBPA-induced alterations in Cyp1a expression, ROS levels, DNA damage, the expression of chondrogenesis-related markers (Sox10 and Sox2), and epithelial to mesenchymal transition (EMT) markers (E-cadherin, N-cadherin, and Snail2). Furthermore, Ahr2 knockdown rescued several TBBPA-induced alterations in craniofacial cartilage parameters. Co-exposure of TBBPA with the antioxidant NAC rescued the ROS and DNA damage and rescued only ceratohyal cartilage length, whereas other cartilage parameters remained disrupted. These findings establish a causal role for the Ahr-Cyp1a axis in TBBPA-induced craniofacial cartilage developmental toxicity and identify ROS as an important downstream contributor, indicating that both ROS-dependent and ROS-independent Ahr mechanisms underlined the observed TBBPA induced craniofacial defects.