title: Aroclor 1254 as a 2,3,7,8-tetrachlorodibenzo-p-dioxin antagonist: effects on enzyme induction and immunotoxicity.
abstract: 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and Aroclor 1254 induced the cytochrome P-450 dependent monooxygenases, aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) in rat hepatoma H-4-II E cells and C57BL/6J mice. It has been proposed that both Aroclor 1254 and 2,3,7,8-TCDD induce these enzymes via a common mechanism which features initial binding to the aryl hydrocarbon (Ah) cytosolic receptor protein. The major difference between these compounds was the relative potency (i.e. 2,3,7,8-TCDD much greater than Aroclor 1254). Cotreatment of rat hepatoma H-4-II E cells or C57BL/6J mice with a dose of 2,3,7,8-TCDD which submaximally induces AHH and EROD and a dose of Aroclor 1254 which exhibited little or no induction activity resulted in significant antagonism of the induction effects of 2,3,7,8-TCDD. For example, cotreatment of C57BL/6J mice with 2,3,7,8-TCDD (15 nmol/kg) and Aroclor 1254 (25, 75 and 150 mumol/kg) resulted in up to 23% antagonism of AHH induction by 2,3,7,8-TCDD. Moreover, cotreatment with a higher dose of the 2,3,7,8-TCDD agonist (30 or 50 nmol/kg) partially reversed some of the antagonism by Aroclor 1254. In vivo antagonism was observed only at Aroclor 1254/2,3,7,8-TCDD molar ratios of 1667:1, 5000:1 and 10,000:1. Administration of 2,3,7,8-TCDD (3.72 nmol/kg) to C57BL/6J mice resulted in a 76% decrease in the splenic plaque forming cell response to sheep red blood cells. This T-cell mediated immunotoxic effect of 2,3,7,8-TCDD segregates with the Ah locus. In contrast, administration of 5, 15, 75 and 150 mumol/kg of Aroclor 1254 resulted in impairment of the immune response only at the highest dose level. However, cotreatment of mice with 2,3,7,8-TCDD (3.72 nmol/kg) and Aroclor 1254 (5, 15 or 75 mumol/kg) resulted in no significant decrease in the plaque forming cell response and complete protection from the immunotoxicity of 2,3,7,8-TCDD. Cotreatment of the mice with Aroclor 1254 (75 mumol/kg) and a higher dose of the 2,3,7,8-TCDD agonist resulted in partial reversal of the protective effects of Aroclor 1254. The in vitro and in vivo data suggest that within specific antagonist/agonist dose ratios, Aroclor 1254 can antagonize at least 2 Ah receptor-mediated effects of 2,3,7,8-TCDD, namely AHH induction and immunotoxicity.
mesh_terms: D000818:Animals; D001140:Aroclors / Q000494:pharmacology*; D001189:Aryl Hydrocarbon Hydroxylases / Q000096:biosynthesis; D020111:Chlorodiphenyl (54% Chlorine); D019363:Cytochrome P-450 CYP1A1; D004147:Dioxins / Q000037:antagonists & inhibitors*; D004305:Dose-Response Relationship, Drug; D004790:Enzyme Induction / Q000187:drug effects; D006462:Hemolytic Plaque Technique; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D010088:Oxidoreductases / Q000096:biosynthesis; D011078:Polychlorinated Biphenyls / Q000494:pharmacology*; D000072317:Polychlorinated Dibenzodioxins / Q000037:antagonists & inhibitors*; D051381:Rats; D013601:T-Lymphocytes / Q000187:drug effects / Q000276:immunology
chemical_list: D001140:Aroclors; D004147:Dioxins; D000072317:Polychlorinated Dibenzodioxins; D020111:Chlorodiphenyl (54% Chlorine); D011078:Polychlorinated Biphenyls; D010088:Oxidoreductases; D001189:Aryl Hydrocarbon Hydroxylases; D019363:Cytochrome P-450 CYP1A1