Expression and inducibility of aryl hydrocarbon receptor pathway genes in wild-caught Killifish (Fundulus heteroclitus) with different contaminant-exposure histories

Wildcaught killifish from a contaminated site on the Elizabeth River (VA, USA) are refractory to induction of cytochrome P4501A (CYP1A, measured as catalytic activity and immunodetectable CYP1A protein) after exposure to typical aryl hydrocarbon receptor (AHR) agonists, as has been reported for fish from other sites highly contaminated with these compounds. In an attempt to understand the molecular basis for the lack of inducibility of CYP1A protein expression and activity in Elizabeth River killifish, we analyzed the expression of CYP1A and four other members of the AHR signal transduction pathway: AHR1, AHR2, AHR repressor (AHRR), and AHR nuclear translocator (ARNT). Gene expression was measured by cycle-optimized reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of messenger ribonucleic acid (mRNA) extracted from livers of killifish from the Elizabeth River and King's Creek (VA, USA) (reference site), 36 h after injection with β-naphthoflavone (BNF, an AHR agonist) or corn oil (carrier control). Hepatic CYP1A mRNA was inducible in King's Creek killifish. However, in Elizabeth River killifish, no induction of CYP1A mRNA was observed, confirming and extending previous results showing no induction of CYP1A protein or catalytic activity in this population. Similarly, AHRR and AHR2 mRNA levels were induced by BNF in King's Creek but not Elizabeth River killifish. No population or treatment-related differences were observed in expression of AHR1 or ARNT mRNAs. The results reveal in Elizabeth River killifish a consistent lack of inducibility of genes that are normally inducible by AHR agonists (CYP1A, AHRR, AHR2). However, the expression of AHR1, AHR2, and AHRR in vehicle-treated fish did not differ between Elizabeth River and King's Creek killifish, suggesting that altered constitutive expression of AHRs or AHRR does not underlie the refractory CYP1A phenotype in Elizabeth River killifish.