Aryl hydrocarbon (Ah) receptor-independent induction of Cypla2 gene expression by acenaphthylene and related compounds in

2To whom correspondence should be addressed Treatment of B6C3F1 mice with acenaphthylene, acenaph-thene, fluorene, phenanthrene, anthracene and dibenzofu-ran resulted in induction of hepatic microsomal methoxy-resorufin 0-deethylase (MROD) activity. Acenaphthylene was the most potent inducer of MROD, a Cyplai-depend-ent activity, and was utilized as a prototypical inducer for this group of tricyclic hydrocarbons. Acenaphthylene (300 mg/kg) caused a> 80-fold induction of hepatic microsomal MROD activity; no induction was observed in kidney or lung. Analysis of induced hepatic microsomes with antibodies to Cyplal and Cypla2 showed that acenaphthy-lene induced immunoreactive Cypla2 but not Cyplal proteins and subsequent mRNA analysis confirmed with a cDNA probe for Cyplal and Cypla2 that acenaphthylene induced Cypla2 but not Cyplal mRNA. Results from nuclear run-on experiments using hepatic nuclei showed that acenaphthylene caused an-4-fold increase in the rate of Cypla2 gene transcription in B6C3F1 mice. Results of competitive binding studies indicated that the tricyclic hydrocarbons did not competitively displace [3H]2r3,7,8-tetrachlorodibenzo-/?-dioxin or [3H]benzo[a]pyrene from the mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor or 4S carcinogen binding protein respectively. The data indicate that acenaphthylene and related tricyclic hydrocar-bons induce Cypla2 gene expression in B6C3F1 mice via an Ah receptor-independent pathway. Thus, tricyclic hydrocarbons induce Cypla2 without the co-induction of Cyplal and therefore these relatively non-toxic compounds can be used to further probe the role of Cypla2 in the metabolism and metabolic activation of diverse chemical carcinogens