2,3,7,8-tetrachlorodibenzo-p-dioxin induction of cytochrome P450- dependent arachidonic acid metabolism in mouse liver microsomes: Evidence for species-specific differences in responses
Cytochrome P-450 Enzyme System
Arachidonic acid is biotransformed to metabolites active in signal transduction 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 CYP-dependent arachidonic acid epoxygenation in chicks but depress epoxygenation in rat liver microsomes where they elicit about twofold increases in formation of other CYP products, omega-1 to omega-4-OH arachidonic acid. These studies examined the effect of TCDD on metabolism of [1-14C]-labeled arachidonic 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 products (EETs and EET-diols) and omega-OH arachidonic acid. Treatment with TCDD 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 formation of epoxygenase products and HETEs each about twofold. In contrast, TCDD treatment decreased formation of omega-OH arachidonic acid by over 70%. EET-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 [1-14C]-labeled arachidonic acid was examined in homogenates of spleen, an immune system target of TCDD. While levels of total [1-14C]-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 Cyp1a1 and 1a2 share immunologic epitopes with chick CYP1A4 and 1A5. However, in immunoinhibition studies, an antibody to CYP1A5, the chick arachidonate epoxygenase, was ineffective against TCDD-induced arachidonic acid metabolism in mouse liver microsomes, suggesting that there are differences in the catalytic sites or tertiary structures of CYP1A5 and the CYP-enzyme catalyzing the TCDD-induced arachidonic acid metabolism in mouse liver. This study shows that the effects of TCDD of the profile of CYP-dependent arachidonic acid metabolities and the amounts produced in mouse liver microsomes differ from other species. The findings suggest that species differences in CYP1A catalytic activities including the metabolism of arachidonic acid may contribute to species differences in sensitivity to TCDD toxicity.