The potential non-additive interactions of polynuclear aromatic hydrocarbon (PAH) mixtures as inducers of Cypla-l and Cypla-2 gene expression were investigated in B6C3F1 mice using a reconstituted PAH mixture. The chemical composition (% by weight) of the reconstituted PAH mixture was: 2-ring PAHs - indan (0.22), naphthalene (23.8), 2-methylnaphthalene (23.2) and l-methylnaphthalene (13.3); 3-ring PAHs - acenaphthylene (7.7), acenaphthene (0.6), dibenzofuran (0.7), fluorene (4.3), phenanthrene (10.5) and anthracene (3.4); greater than or equal to 4-ring PAHs - fluoranthene (2.4), pyrene (4.3), benz[a]anthracene (1.4), chrysene (1,5), benzo[b]fluoranthene (0.8), benzo[k]fluoranthene (0.9) and benzo[a]pyrene (0.9). The composition of the 2-, 3- and greater than or equal to 4-ring PAH fractions were based on the relative concentration of individual PAHs as noted above, The greater than or equal to 4-ring PAH fraction and reconstituted mixture induced hepatic microsomal ethoxyresorufin O-deethylase (EROD) activity and Cypla-l mRNA levels, whereas the 2- and 3-ring PAHs were only weakly active. Direct comparison of the potencies of the reconstituted mixture and greater than or equal to 4-ring PAHs showed that the Cypla-1 induction activity of the reconstituted mixture was due to the greater than or equal to 4-ring PAHs, The reconstituted PAH mixture and greater than or equal to 4-ring PAHs also induced Cypla-2 hepatic mRNA levels and microsomal methoxyresorufin O-deethylase (MROD) activity; however, their dose-response curves indicated that the reconstituted PAH mixture was more potent as a Cypla-2 inducer than the greater than or equal to 4-ring PAHs. The differences in potency were due to 3-ring PAHs which were found to be strong inducers of hepatic Cypla-2 mRNA levels and microsomal MROD activity at the lowest dose administered (37 mg/kg). The 3-ring mixture caused a maximal 29-fold increase in hepatic MROD activity at a dose of 292 mg/kg, but only 28% of maximal induction of EROD activity. Northern analysis of liver mRNA from mice treated with 3-ring PAHs showed that there was minimal induction of Cypla-1 mRNA levels. The 3-ring PAHs did not competitively bind to the mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor suggesting that 3-ring PAHs are a new class of Cypla-2 inducers which do not act through the Ah receptor.
