Astroff B, Zacharewski T, Safe S, Arlotto M P, Parkinson A, Thomas P, Levin W
Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine, Texas A & M University, College Station 77843.
Mol Pharmacol. 1988 Feb;33(2):231-6.
In addition to being one of the most toxic chemicals known, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent inducer of rat liver microsomal cytochrome P-4501A1 (P-450c). Previous studies have demonstrated that a high affinity, low capacity cytosolic receptor (the Ah receptor) mediates the activity of TCDD to induce cytochrome P-4501A1, which catalyzes benzo[a]pyrene hydroxylation [aryl hydrocarbon hydroxylase (AHH]) and 7-ethoxyresorufin O-dealkylation (EROD). The results of the present study indicate that 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) effectively competes with [3H]TCDD for binding to the Ah receptor in rat liver cytosol. The concentration of MCDF effecting 50% displacement of [3H]TCDD was 4.9 X 10(-8) M, which is approximately 50 times greater than the EC50 for unlabeled TCDD (approximately 1 X 10(-9) M). However, in contrast to TCDD, MCDF was only a weak inducer of AHH and EROD activity in rat hepatoma H-4-II cells in culture. When co-incubated, MCDF diminished in a concentration-dependent manner the ability of TCDD to induce AHH and EROD activity in vitro. Treatment of rats with 20-200 mumol/kg MCDF in vivo had little or no effect on liver microsomal AHH and EROD activity, whereas treatment of rats with 16 nmol/kg TCDD caused a 6- and a 70-fold induction of AHH and EROD activity, respectively. When co-administered, MCDF diminished by approximately 50% the ability of TCDD to induce AHH and EROD activity in vivo. The partial antagonism produced by 50 mumol/kg MCDF could be partially overcome by doubling the dosage of TCDD from 16 to 32 nmol/kg. Immunochemical analysis of rat liver microsomes revealed that treatment of rats with 20-200 mumol/kg MCDF caused little or no induction of cytochromes P-4501A1 and P-4501A2 (P-450d), whereas these isozymes were induced 33- and 5-fold, respectively, in rats treated with 16 nmol/kg TCDD. When co-administered, MCDF diminished by approximately 50% the ability of TCDD to induce cytochrome P-4501A1 in vivo, which established that MCDF was not simply acting as an inhibitor of AHH and EROD activity. MCDF also antagonized the ability of TCDD to induce cytochrome P-4501A2, which suggests that the induction of both cytochromes P-4501A1 and P-4501A2 is regulated by the Ah receptor. These results indicate that MCDF binds with high affinity to the Ah receptor in rat liver cytosol and competitively blocks the binding of TCDD.(ABSTRACT TRUNCATED AT 400 WORDS)
2,3,7,8-四氯二苯并-对-二恶英（TCDD）不仅是已知毒性最强的化学物质之一，还是大鼠肝微粒体细胞色素P - 4501A1（P - 450c）最有效的诱导剂。以往研究表明，一种高亲和力、低容量的胞质受体（芳烃受体）介导TCDD诱导细胞色素P - 4501A1的活性，该细胞色素催化苯并[a]芘羟基化[芳烃羟化酶（AHH）]和7 - 乙氧基异吩恶唑酮O - 脱烷基作用（EROD）。本研究结果表明，6 - 甲基 - 1,3,8 - 三氯二苯并呋喃（MCDF）能有效地与[³H]TCDD竞争结合大鼠肝细胞质中的芳烃受体。使[³H]TCDD 50%被置换的MCDF浓度为4.9×10⁻⁸ M，约为未标记TCDD的半数有效浓度（EC50，约1×10⁻⁹ M）的50倍。然而，与TCDD不同，MCDF在培养的大鼠肝癌H - 4 - II细胞中只是AHH和EROD活性的弱诱导剂。共同孵育时，MCDF以浓度依赖的方式降低TCDD在体外诱导AHH和EROD活性的能力。给大鼠体内注射20 - 200 μmol/kg的MCDF对肝微粒体AHH和EROD活性几乎没有影响，而给大鼠注射16 nmol/kg的TCDD分别使AHH和EROD活性诱导了6倍和70倍。共同给药时，MCDF使TCDD在体内诱导AHH和EROD活性的能力降低约50%。50 μmol/kg的MCDF产生的部分拮抗作用可通过将TCDD剂量从16 nmol/kg加倍至32 nmol/kg而部分克服。对大鼠肝微粒体的免疫化学分析显示，给大鼠注射20 - 200 μmol/kg的MCDF几乎不诱导细胞色素P - 4501A1和P - 4501A2（P - 450d），而在注射16 nmol/kg TCDD的大鼠中，这些同工酶分别被诱导了33倍和5倍。共同给药时，MCDF使TCDD在体内诱导细胞色素P - 4501A1的能力降低约50%，这表明MCDF并非仅仅作为AHH和EROD活性的抑制剂起作用。MCDF还拮抗TCDD诱导细胞色素P - 4501A2的能力，这表明细胞色素P - 4501A1和P - 4501A2的诱导均受芳烃受体调控。这些结果表明，MCDF与大鼠肝细胞质中的芳烃受体具有高亲和力结合，并竞争性地阻断TCDD的结合。（摘要截选至400字）
