Competitive binding affinity of carcinogenic aromatic amines to the rat hepatic aromatic hydrocarbon (ah) receptor in vitro and potency to induce monooxygenase activity in vivo

The aromatic amides 2-acetylaminofluorene (2-AAF), its isomer 4-acetylaminofluorene, 2-acetylaminophenanthrene (AAP), trans-4-acetylaininostilbene (AAS) and the corresponding amines differ in their carcinogenic effects in the rat. Only 2-AAF and 2-aminofluorene (2-AF}) are tumor promoters in rat liver. We have determined the competitive binding affinities of these compounds to the rat hepatic cytosolic aromatic hydrocarbon (Ah) receptor, which has been associated with tumor promotion. Binding affinity was determined by displacement of labelled 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD) or 3-methylcholanthrene. The rank order of affinities and the apparent inhibitor constants measured with the sucrose density gradient centrifugation technique were: AAF (2.3 μM) > AAP (2.7 μM) > 2-AF (7.0 μM) > trans-4-aminostilbene (7.7 μM) > 2-amninophen- anthrene (10.4 μM). 4-Acetylaminofluorene, 4-aminofluorene and AAS did not displace TCDD from the receptor protein. A number of other fluorene derivatives did not bind, except 2-nitrofluorene (apparent inhibitor constant (1.4 μM). The in vivo biological potency of the aromatic amines was monitored by measuring the induction of ethoxy resorufin-O-deethylase (EROD) and aromatic hydrocarbon hydroxylase activities. In vitro binding to the Ah receptor correlates roughly with the induction of EROD activity. The effect of 2-AAF was dose dependent, supporting a receptor- mediated mechanism. The correlation between Ah receptor affinity and tumor-promoting properties of the tested aromatic amines is only limited, but the results are not at variance with the concept of a receptor-mediated process in 2-AAF tumor promotion.