Multiple mechanisms of PAHs toxicity in Daphnia magna: effects of an AhR agonist and competitor on genes involved in toxicant pathways (1138.11)

Abstract

Understanding the mechanistic pathways underlying the toxicity of chemicals can provide more biologically relevant models to extrapolate adverse outcomes. Mechanistically based models should provide more efficient means to screen untested compounds and predict exposure to untested species. We evaluated the relationship between reproductive outcomes in Daphnia magna and expression of biomarkers after exposure to polyaromatic hydrocarbons (PAHs). These biomarkers include Daphnia magna homologs of ARNT (aryl hydrocarbon nuclear translator), CYP4 (Cytochrome P450 Family 4), COX (cyclooxygenase), retinoid‐X‐receptor (RXR), and juvenile hormone esterase (JHE). To assess physiological endpoints we monitored reproductive changes in a 14‐day chronic exposure. Exposure to benzo(a)pyrene resulted in altered reproduction and was correlated with increases in RXR and decreases in VTG and CYP4 genes. We then conducted exposures with a known cytochrome P450 inhibitor, PBO (piperonyl butoxide and phenanthrene. PBO alone induced ARNT, JHE, COX, RXR, and CYP4 more than phenanthrene alone. In combination with PBO, phenanthrene did not induce JHE, RXR, and downregulated phase I detoxification pathway genes ARNT and CYP4. CYP450 activity (ARNT) does not have a role in induction of developmental hormones (JHE). These results indicate that Daphnia toxicant control mechanisms are operating through multiple pathways that influence reproduction and survival.