AHR2-mediated transcriptomic responses underlying the synergistic cardiac developmental toxicity of PAHs

Polycyclic aromatic hydrocarbons (PAHs) induce developmental defects including cardiac deformities in fish. The aryl hydrocarbon receptor (AHR) mediates the toxicity of some PAHs. Exposure to a simple PAH mixture during embryo development consisting of an AHR agonist (benzo(a)pyrene-BaP) with fluoranthene (FL), an inhibitor of cytochrome p450 1(CYP1) - a gene induced by AHR activation- results in cardiac deformities. Exposure to BaP or FL alone at similar concentrations alters heart rates, but does not induce morphological deformities. Furthermore, AHR2 knockdown prevents the toxicity of BaP+FL mixture. Here we used a zebrafish microarray analysis to identify heart-specific transcriptomic changes during early development that might underlie cardiotoxicity of BaP+FL. We used AHR2 morphant embryos to determine the role of this receptor in mediating toxicity. Control and knockdown embryos at 36 hours post fertilization were exposed to DMSO, 100 μg/L BaP, 500 μg/L FL, or 100 μg/L BaP + 500 μg/L FL, and heart tissues for RNA were extracted at 2, 6, 12, and 18 hours-post-exposure (hpe), prior to the appearance of cardiac deformities. Data show AHR2-dependent BaP+FL effects on expression of genes involved in protein biosynthesis and neuronal development in addition to signaling molecules and their associated molecular pathways. Ca2+-cycling and muscle contraction genes were the most significantly differentially expressed category of transcripts when comparing BaP+FL-treated AHR2 morphant and control embryos. These differences were most prominent at 2 and 6 hpe. Therefore, we postulate that BaP+FL may affect cellular Ca2+ levels and subsequently cardiac muscle function, potentially underlying BaP+FL cardiotoxicity.