Structure-dependent induction of CYP2B by polychlorinated biphenyl congeners in female Sprague-Dawley rats

Abstract

The dose-response induction of hepatic microsomal pentoxyresorufin O-dealkylase (PROD) activity by phenobarbital (PB) and several polychlorinated biphenyl (PCB) mixtures and congeners was determined in the immature female Sprague-Dawley rat. At a dose of 75 mg/kg/day of PB for 3 days, the microsomal PROD activity was 2154 pmol/min/mg protein. Aroclors 1260, 1254, 1242, and 1016 did not induce maximal PROD activity at doses up to 500 mg/kg, and only Aroclor 1016 induced > a half-maximal response at the 500 mg/kg dose. The relative potencies of eighteen different PCB congeners were also determined, and the structures of these compounds differed with respect to the degree of chlorination (tri- to octochloro) and substitution patterns. The relative potencies of these compounds were estimated by comparing their induced activities at the high dose (150 or 100 mg/kg) with that of PB. The most potent inducers were 2,3,3′,4′,5,6-hexaCB and 2,2′,3,4′,5,5′,6-heptaCB; at a dose of 150 mg/kg, the PROD activity induced by 2,2′,3,4′,5,5′,6-heptaCB was comparable to that observed for PB. 2,3,3′,4′,5,6-HexaCB was the most potent inducer, and hepatic PROD activity in rats treated with 150 mg/kg was 4202 pmol/min/mg; this value was higher than that observed for PB at a dose of 75 mg/kg. A second group of congeners including 2,2′,3,4,4′,5,5′-heptaCB, 2,2′,4,4′,5,5′-hexaCB, 2,2′,3,3′,4,4′,5,5′-oc-taCB, 2,2′,4,4′-tetraCB, 2,2′,4,5,5′-pentaCB, 2,2′,3,4,4′,5′,6-heptaCB, 2,2′,4,4′,5-pentaCB and 2,2′,3,3′,4′,5,5′,6-octaCB induced PROD activity ⩾ 1090 pmol/min/mg at the 150 mg/kg dose, and this value was > 50% of the maximal response observed for PB. The remaining compounds, namely 2,4,4′-triCB, 2,2′,3,4′-tetraCB, 2,2′,5,5′-tetraCB, 2,3′,4,4′,5-pentaCB, 2,3,3′,4,4′-pentaCB, 2,2′,4,4′,5,6′-hexaCB, 2,3,3′,4,4′,5,5′-heptaCB and 2,2′,3,-3′,4,4,5-heptaCB were all relatively weak inducers of hepatic microsomal PROD activity (< 450 pmol/min/mg). In parallel experiments, western blot analysis of immunoreactive CYP2B1 and CYP2B2 proteins showed that PB, the PCB mixtures, and congeners induced both proteins. Previous studies have identified a cis-acting DNA element that plays a role in regulating CYP2B1/B2 gene expression and binds nuclear trans-acting factor(s) induced by PB. The results of gel electrophoretic mobility shift assays with nuclear extracts showed that both PB and 2,2′,3,4′,5,5′,6-heptaCB induce formation of a common retarded band using a 32P-labeled oligonucleotide corresponding to the cis-acting DNA promoter sequence. Both PB and PCBs appear to induce CYP2B1/B2 via a common mechanism. Although the results of this study do not define structure-induction (CYP2B1/B2) relationships for PCBs, two compounds, namely 2,3,3′,4′,5,6-hexaCB and 2,2′,3,4′,5,5′,6-heptaCB, were identified as highly potent inducers.