2,2′,4,4′,5,5′-Hexachlorobiphenyl as a 2,3,7,8-tetrachlorodibenzo-p-dioxin antagonist in C57BL 6J mice

At doses as high as 750 to 1000 μmol/kg, 2,2′,4,4′,5,5′-hexachlorobiphenyl (HCBP) did not cause fetal cleft palate, suppress the splenic plaque-forming cell response to sheep red blood cells, or induce hepatic microsomal ethoxyresorufin O-deethylase (EROD) in C57BL 6J mice. Despite the lack of activity of HCBP in eliciting any of these aryl hydrocarbon (Ah) receptor-mediated responses, competitive binding studies indicated that HCBP competitively displaced 2,3,7,8-[³H]tetrachlorodibenzo-p-dioxin (TCDD) from the murine hepatic cytosolic receptor. Cotreatment of C57BL 6J mice with TCDD (3.7 nmol/kg) and HCBP or 4,4′-diiodo-2,2′,5,5′-tetrachlorobiphenyl (I₂-TCBP) (400 or 1000 μmol/kg) showed that both compounds partially antagonized TCDD-mediated cleft palate and immunotoxicity (i.e., suppression of the splenic plaque-forming cell response to sheep red blood cells), and HCBP antagonized TCDD-mediated hepatic microsomal EROD induction. Thus, HCBP and I₂-TCBP, like the commercial polychlorinated biphenyl mixture Aroclor 1254, were partial antagonists of TCDD action in C57BL 6J mice; however, it was also apparent from the results that Aroclor 1254 was the more effective antagonist at lower doses. Using [³H]TCDD, it was also shown that some of the effects of HCBP on TCDD-mediated cleft palate may be due to the decreased levels of TCDD found in the fetal palates after cotreatment with TCDD and HCBP. 4,4′-[¹²⁵I₂]diiodo-2,2′,5,5′-tetrachlorobiphenyl ([¹²⁵I₂]TCBP) of high specific activity (3350 Ci/mmol) was synthesized and used to investigate the direct binding of this compound to the murine hepatic Ah receptor or other cytosolic proteins. No direct specific binding was observed between ¹²⁵I₂-TCBP and any cytosolic proteins using a sucrose density gradient assay procedure. These results contrasted with previous studies with Aroclor 1254 that suggested that this mixture acted as a competitive Ah receptor antagonist.