Biphasic response for hepatic microsomal enzyme induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin in C57BL/6J and...

The induction of the murine hepatic microsomal cytochrome P-450 monooxygenase system by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied over a wide range of doses, including those associated with acute toxicity. Studies were conducted in two inbred strains of mice which vary at the Ah receptor and at a number of other genetic loci. C57BL/6J mice possess a high-affinity Ah receptor and are responsive to enzyme inductive effects of TCDD, whereas DBA/2J mice do not possess a high-affinity receptor and are less responsive to TCDD. In a dose-response study, 7-ethoxyresorufin O-deethylase (EROD) activity appeared to be maximally induced in C57BL/6J and DBA/2J mice at 7 days following exposure to 3 and 30 micrograms of TCDD/kg respectively. Very similar results were reported previously for the induction of aryl hydrocarbon hydroxylase activity in these strains of mice. However, at higher doses of TCDD (at least 45 micrograms/kg for C57BL/6J and 300 micrograms/kg for DBA/2J), EROD activity was further increased (2-fold) from the apparent maximal (plateau) level, resulting in an unusual biphasic log dose-response relationship. EROD activity remained at these elevated rates in both strains for doses approaching and exceeding the respective LD50 values for each strain. To further characterize this biphasic induction phenomenon, cytochrome P-450 content, benzo[a]pyrene metabolism, and EROD and NADPH-cytochrome P-450 reductase activities were measured 1, 3 and 7 days after TCDD administration to C57BL/6J (3 and 150 micrograms/kg) and DBA/2J (30 and 600 micrograms/kg) mice. Maximal responses occurred in both strains at 3 days for all doses. In both strains, TCDD produced a dose-dependent increase in cytochrome P-450 content, EROD, and benzo[a]pyrene metabolism. Furthermore, a 2-fold induction of reductase activity was observed in each strain following exposure to the respective high doses. Induction of cytochrome P1-450 and P3-450 was also measured by Western immunoblot, using antisera raised against the homologous rat isozymes. In both strains, TCDD produced a dose-related increase in two protein-staining bands recognized by anti-P-450BNF-B (P1-450) and anti-P-450BNF/ISF-G (P3-450) respectively. The extended induction of hepatic microsomal monooxygenase activities at the respective high doses of TCDD appears to be due, in part, to increases in NADPH-cytochrome P-450 reductase activity and cytochromes P1-450 and P3-450 content. Significant alterations in the expression of the cytochrome P-450 monooxygenase system following exposure to high doses of TCDD may be associated, in part, with the delayed acute toxicity reported at this level of exposure.