Mullen Grey Anne K, Riddick David S
Department of Pharmacology and Toxicology, Medical Sciences Building, University of Toronto, Toronto, Ontario, Canada M5S 1A8.
Chem Biol Interact. 2009 Dec 10;182(2-3):148-58. doi: 10.1016/j.cbi.2009.07.005. Epub 2009 Jul 16.
The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the effects of aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3-methylcholanthrene (MC); the prototypical response is induction of drug-metabolizing enzymes. Factors that regulate AHR levels in vivo are poorly understood and it is also not clear how AHR levels affect aromatic hydrocarbon responsiveness. Our interest in pituitary-dependent regulation of AHR levels was prompted by two findings from our laboratory: (1) hypophysectomized rats have reduced hepatic levels of AHR protein; and (2) glucocorticoids increase AHR expression and aromatic hydrocarbon responsiveness in rodent hepatoma cells. To study whether adrenalectomy and glucocorticoids contribute to hormone-dependent regulation of the hepatic AHR pathway, male adrenalectomized (ADX) or SHAM-ADX rats were treated with dexamethasone (DEX) or vehicle. AHR protein was depleted by 50-60% at 4 days after ADX, but was not altered by DEX treatment. To assess whether the observed AHR depletion affected aromatic hydrocarbon responsiveness, the induction of hepatic cytochrome P450 1B1 (CYP1B1) mRNA by MC was measured as an AHR-mediated adaptive response. MC-induced hepatic CYP1B1 mRNA was reduced by 50% in ADX rats relative to SHAM-ADX. Exogenous glucocorticoid treatment (DEX - 1.5mg/kg) induced hepatic AHR nuclear translocator (ARNT) mRNA by up to 9-fold at 3 and 6h after dosing, with no corresponding change in ARNT protein levels. These data demonstrate that: (1) adrenal-dependent factors contribute to the physiological maintenance of hepatic AHR protein levels; (2) the depletion of hepatic AHR protein in ADX rats coincided with a diminished adaptive response to MC; and (3) exogenous glucocorticoid treatment increases hepatic ARNT mRNA levels regardless of adrenal status. This model is useful for studying the mechanisms of AHR and ARNT regulation and for further characterization of the impact of AHR protein depletion on the response to aromatic hydrocarbons in vivo.
芳烃受体（AHR）是一种配体激活的转录因子，介导芳烃如2,3,7,8 - 四氯二苯并 - p - 二恶英和3 - 甲基胆蒽（MC）的效应；典型反应是诱导药物代谢酶。体内调节AHR水平的因素了解甚少，AHR水平如何影响芳烃反应性也不清楚。我们实验室的两项发现促使我们关注垂体对AHR水平的依赖性调节：（1）垂体切除的大鼠肝脏中AHR蛋白水平降低；（2）糖皮质激素可增加啮齿动物肝癌细胞中AHR的表达及芳烃反应性。为研究肾上腺切除和糖皮质激素是否参与肝脏AHR途径的激素依赖性调节，对雄性肾上腺切除（ADX）或假手术（SHAM - ADX）大鼠给予地塞米松（DEX）或赋形剂处理。ADX术后4天，AHR蛋白减少50 - 60%，但DEX处理未改变其水平。为评估观察到的AHR减少是否影响芳烃反应性，以MC诱导肝脏细胞色素P450 1B1（CYP1B1）mRNA作为AHR介导的适应性反应进行测定。相对于SHAM - ADX大鼠，ADX大鼠中MC诱导的肝脏CYP1B1 mRNA减少50%。外源性糖皮质激素处理（DEX - 1.5mg/kg）在给药后3小时和6小时可使肝脏AHR核转运体（ARNT）mRNA增加至9倍，而ARNT蛋白水平无相应变化。这些数据表明：（1）肾上腺依赖性因素有助于维持肝脏AHR蛋白水平的生理状态；（2）ADX大鼠肝脏AHR蛋白的减少与对MC的适应性反应减弱相一致；（3）外源性糖皮质激素处理可增加肝脏ARNT mRNA水平，而与肾上腺状态无关。该模型对于研究AHR和ARNT的调节机制以及进一步表征AHR蛋白减少对体内芳烃反应性的影响很有用。
