Induction of EROD activity by 1-phenylimidazole and β-naphthoflavone in rainbow trout cultured hepatocytes: A comparative study

The classical pathway for induction of cytochrome P4501A (CYP1A) by xenobiotics is ligand binding to the aryl hydrocarbon receptor (AhR). However, several studies with mammalian cell systems point out a range of xenobiotics including imidazole derivatives, which are able to activate CYP1A through non-classical mechanisms. The objective of the present work is to compare induction of CYP1A (determined at the catalytic level as 7-ethoxyresorufin-O-deethylase, EROD) in rainbow trout (Oncorhynchus mykiss) hepatocytes by the prototypic AhR ligand, β-naphthoflavone (βNF), and by the imidazole derivative, 1-phenylimidazole (PIM). PIM was able to induce EROD activity although its potency was clearly lower than that of βNF. In order to assess the relative importance of classical AhR ligand binding and alternative signaling pathways in CYP1A induction by PIM, co-exposure experiments with the partial AhR antagonist α-naphthoflavone (αNF) or with inhibitors of protein kinase C (staurosporine) and tyrosine kinases (genistein, herbimicine) were performed. αNF and herbimicin provoked a decrease of EROD induction both by βNF and PIM, whereas staurosporine and genistein remained without effect. The overall similarities in the response of βNF and PIM to the various inhibitors suggest that both compounds, in apparent contrast to the behaviour of some other imidazole derivatives, induce CYP1A following similar mechanisms.