Evidence for multiple mechanisms of toxicity in larval rainbow trout (Oncorhynchus mykiss) co-treated with retene and alpha-naphthoflavone.
Alkylated polycyclic aromatic hydrocarbons, such as retene (7-isopropyl-1-methylphenanthrene), induce cytochrome P450 1A (CYP1A) enzymes and produce dioxin-like toxicity in the embryo-larval stages of fish characterized by the signs of blue sac disease (BSD). The signs of toxicity are well characterized; however, the mechanism is not well understood. To elucidate the role of CYP1A in retene toxicity, larval rainbow trout (Oncorhynchus mykiss) were co-treated with a range of concentrations of alpha-naphthoflavone (ANF), a known CYP1A inhibitor. The co-treatment produced synergistic toxicity at 3.2-100 microg/L ANF, after which toxicity at 180 microg/L ANF dropped to levels typical of retene-only. At 320 microg/L ANF, toxicity increased with or without retene, indicating that ANF alone was capable of inducing BSD. In addition, the additive toxicity of retene-only and 320 microg/L ANF-only approximately equalled that of the co-exposed larvae (100 microg/L retene+320 microg/L ANF), indicating response addition. Thus, two mechanisms of action occurred in co-exposed larvae at different concentrations of ANF. In trout larvae, there was a correlation between toxicity and CYP1A protein concentrations, and in juvenile trout, ANF produced a concentration-dependent inhibition of ethoxyresorufin-O-deethylase (EROD) activity without a measurable drop in CYP1A protein. Taken together, the mechanism underlying the synergistic toxicity is EROD-independent and may be AhR-dependent. This study demonstrated that multiple, exposure-dependent mechanisms can occur in mixture toxicity, suggesting that current risk assessment models may drastically underestimate toxicity, particularly of mixtures containing both CYP1A inducers and inhibitors.
Department of Biology, Queen's University, Kingston, ON, Canada.
Jason A
JA
Scott
Peter V
PV
Hodson
D000818:Animals; D001571:Benzoflavones / Q000633:toxicity*; D015153:Blotting, Western; D019363:Cytochrome P-450 CYP1A1 / Q000032:analysis / Q000037:antagonists & inhibitors / Q000378:metabolism; D005393:Fish Diseases / Q000139:chemically induced* / Q000201:enzymology; D008099:Liver / Q000201:enzymology / Q000378:metabolism; D017686:Oncorhynchus mykiss; D010616:Phenanthrenes / Q000633:toxicity*; D018336:Receptors, Aryl Hydrocarbon / Q000378:metabolism; D018709:Statistics, Nonparametric; D016019:Survival Analysis; D018675:Toxicity Tests
D001571:Benzoflavones; D010616:Phenanthrenes; D018336:Receptors, Aryl Hydrocarbon; C447880:retene; C011512:alpha-naphthoflavone; D019363:Cytochrome P-450 CYP1A1