α-Naphthoflavone-induced CYP1A1 gene expression and cytosolic aryl hydrocarbon receptor transformation

α-Naphthoflavone (αNF) is a weak aryl hydrocarbon (Ah) receptor agonist and inhibits the induction of CYP1A1 gene expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin. It has been suggested that the Ah receptor antagonist activity is due to the formation of αNF-cytosolic Ah receptor complexes that fail to undergo transformation. This hypothesis is consistent with data obtained in this and other studies using αNF concentrations from 10 to 1000 nM. However, 10 μM αNF exhibited Ah receptor agonist activity in several assays. Incubation of rat hepatic cytosol with 10 μM αNF caused transformation of the Ah receptor, as determined in a gel retardation assay using a ³²P-labeled oligonucleotide containing a single dioxin-responsive element (DRE). Incubation of rat hepatoma (H-4-II E) cells with 10 μM αNF not only resulted in the induction of CYP1A1 mRNA levels but also increased chloramphenicol acetyltransferase activity from a DRE-containing chloramphenicol acetyltransferase reporter plasmid. Moreover, the DRE-transformed cytosolic Ah receptor complex liganded with either αNF or 2,3,7,8-tetrachlorodibenzo-p-dioxin did not undergo significant dissociation at 4°. These data confirm that αNF is an Ah receptor agonist and, based on the results of previous studies, exhibits partial antagonist activity via competition for receptor binding sites.