Relative potencies of induction of hepatic drug-metabolizing enzyme genes by individual PCB congeners.

The induction of a variety of drug-metabolizing enzymes by polychlorinated biphenyl (PCB) congeners that elicit a 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD)-type hepatic pleiotropic response, including 2,3,3',4,4'-pentachlorobiphenyl (BZ 105), 2,3',4,4',5-pentachlorobiphenyl (BZ 118), 2,3,3',4,4',5-hexachlorobiphenyl (BZ 156), and 3,3',4,4',5,5'-hexachlorobiphenyl (BZ 169) was examined. Following dietary exposure to the individual congeners for 5 days, livers were removed and catalytic assays for cytochrome P450 (CYP) isozymes 1A1 and 1A2 were performed. Additionally, total cellular RNA coding for hepatic drug-metabolizing genes (CYP 1A1, CYP 1A2, microsomal epoxide hydrolase, glutathione S-transferase [GST] Ya/Yc, and the TCDD-inducible isozyme of aldehyde dehydrogenase [ALDH] was quantified. 3-Methylcholanthrene (MC), TCDD, or BZ 156 (32 ppm) caused nearly maximal induction of the CYP 1A proteins but lower induction of the other genes. When the dose-response curves for induction of various drug-metabolizing genes (CYP 1A1 and 1A2, microsomal epoxide hydrolase, the GST Ya/Yc subfamily and ALDH) were examined, a spectrum of ED50s (half-maximal inductions) was observed. While CYP 1A2 exhibited an ED50 of 1.7 ppm, the induction of ALDH was shifted far to the right (ED50 > 11 ppm). Thus, different genes in a single tissue may display different dose-response characteristics. The potency (extent of induction of CYP 1A1 activity resulting from a given dietary dose) was BZ 169 >> BZ 156 > BZ 118 > BZ 105. In contrast, the potencies of the four congeners for CYP 1A1 induction were nearly equivalent when related to hepatic PCB burden, apparently due to the preferential accumulation in the liver of BZs 169 and 156 following low-level administration in the diet.