Characterization of a novel cytochrome P450 from the transformable cell line, C3H/10T1/2

The cytochrome P450 in the transformable C3H/10T1/2 (10T1/2) cell line has been characterized and compared to the major polycyclic aromatic hydrocarbon (PAH)-inducible hepatic form, cytochrome P450IA1 (P450IA1). The mouse hepatoma cell line, Hepa-1, was used as an in vitro model for P450IA1 expression and regulation by PAH. Microsomes from uninduced and benz[a]anthracene (BA)-induced 10T1/2 cells provided PAH mono-oxygenated product profiles that were totally different from metabolite profiles produced by microsomes from uninduced and BA-induced Hepa-1 cells even though total activities were similar. The proximate carcinogen, 7,12-dimethylbenz[a]anthracene-3,4-diol (DMBA-3,4-diol) was a major product for the 10T1/2 microsomes, while Hepa-1 formed less than 2% of this metabolite. Hepa-1 converted benzo[a]pyrene (BP) to BP-4,5-diol and DMBA to 7-hydroxymethyl-12-methyl-BA, while 10T1/2 did not produce either product. Polyclonal antibody to rat hepatic P450IA1 did not inhibit metabolism of either PAH substrate by 10T1/2 microsomes, but totally inhibited such metabolism by Hepa-1 microsomes. Western immunoblot analysis of BA-induced 10T1/2 microsomes showed that less than 1% of total P450 was P450IA1. The PAH-metabolizing activity of 10T1/2 microsomes was highly inducible (14-fold) by pre-treatment of non-confluent intact cells with BA, but was only half as inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin. In contrast, the P450IA1 activity of Hepa-1 cells was highly inducible by both compounds. The distinct metabolite profiles, antibody inhibition data and lack of immunoreactivity all indicate that PAH metabolism in 10T1/2 cells is catalyzed by a form of P450 distinct from P450IA1. The anomalous induction patterns suggest that this novel isozyme is predominantly regulated by a mechanism other than the Ah receptor.