2,3,7,8-tetrachlorodibenzo-p-dioxin induction of cytochrome p450-dependentarachidonic acid metabolism in mouse liver microsomes: Evidence for species-specific differences in responses

Arachidonic acid is biotransformed to metabolites active in signal transduc tion by cytochrome P450 (CYP) as well as by cyclooxygenase and lipoxygenase enzymes. Inducers of CYP1 enzymes, including 2,3,7,8-tetrachlorodibenzo-p- dioxin and other Ah receptor ligands, markedly increase liver microsomal CY P-dependent arachidonic acid epoxygenation in chicks but depress epoxygenat ion in rat liver microsomes where they elicit about twofold increases in fo rmation of other CYP products, omega-1 to omega-4-OH arachidonic acid. Thes e studies examined the effect of TCDD on metabolism of [1-C-14]-labeled ara chidonic acid by mouse liver microsomes. Mouse liver microsomes metabolized arachidonic acid exclusively by a CYP-dependent mechanism as evidenced by lack of metabolism in the absence of NADPH and by formation of specific CYP -dependent metabolites. The major constitutive products were epoxygenase pr oducts (EETs and EET-diols) and omega-OH arachidonic acid. Treatment with T CDD increased formation of omega-2- to omega-4-OH arachidonic acid products 23-fold, formation of omega-1-OH arachidonic acid about 5-fold, and format ion of epoxygenase products and HETEs each about twofold. In contrast, TCDD treatment decreased formation of omega-OH arachidonic acid by over 70%, EE T-diols comprised a greater fraction of total epoxygenase products in mouse liver microsomes than has been found for liver microsomes of other species . The high EET-diol formation was attributable to a non-TCDD-inducible, EET epoxide hydrolase activity in mouse liver microsomes. For comparison, the effect of TCDD on [1-C-14]-labeled arachidonic acid was examined in homogen ates of spleen, an immune system target of TCDD, While levels of total [1-C -14]-arachidonic acid metabolism were comparable in both tissues, virtually all of the metabolism by spleen was CYP-independent, and it was unaffected by TCDD, Western blotting experiments showed that TCDD-induced mouse Cyp1a 1 and 1a2 share immunologic epitopes with chick CYP1A4 and 1A5. However, in immunoinhibition studies, an antibody to CYP1A5, the chick arachidonate ep oxygenase, was ineffective against TCDD-induced arachidonic acid metabolism in mouse liver microsomes, suggesting that there are differences in the ca talytic sites or tertiary structures of CYP1A5 and the CYP-enzyme catalyzin g the TCDD-induced arachidonic acid metabolism in mouse Liver, This study s hows that the effects of TCDD of the profile of CYP-dependent arachidonic a cid metabolites and the amounts produced in mouse liver microsomes differ f rom other species. The findings suggest that species differences in CYP1A c atalytic activities including the metabolism of arachidonic acid may contri bute to species differences in sensitivity to TCDD toxicity. (C) 1998 Acade mic Press.