ALTERED- RETINOID AND THYROXIN METABOLIC RESPONSE TO 2,3,7,8-TETRACHLORODIBENSO-p-DIOXIN

为了确定暴露于2,3,7,8-四氯二苯并-对二恶英（TCDD）后甲状腺激素和类视黄醇稳态的破坏是否可以通过芳烃受体（AhR）介导，在妊娠第12.5天给AhR-杂合（AhR+/-）妊娠小鼠单次口服10 μg/kg TCDD。在出生后第21天，收集小鼠血清和肝脏，这些小鼠分别为对照和tcdd处理的AhR+/-和AhR-缺失（AhR-/-）小鼠。虽然TCDD暴露导致AhR+/-小鼠血清中总甲状腺素（TT4）和游离T4 （FT4）水平显著降低，但TCDD对AhR-/-小鼠没有影响。TCDD可显著诱导AhR+/-小鼠肝脏中udp -葡萄糖醛基转移酶（UGT）1A6、细胞色素P450 (CYP) 1A1和CYPIA2的基因表达，但对AhR-/-小鼠无显著影响。免疫组织化学证据证实，CYP1A1蛋白在TCDD诱导下明显定位于小叶中心区肝细胞的细胞质中。暴露于tcdd的AhR+/-小鼠肝脏中棕榈酸视黄醇的水平大大降低，但在载体处理的AhR+/-小鼠中没有。未发现TCDD对AhR-/-小鼠肝脏中类视黄醇水平的影响。我们得出结论，甲状腺激素和类维生素a稳态的破坏完全是通过AhR介导的。UGT1A6的诱导被认为至少在一定程度上导致了tcdd暴露小鼠血清甲状腺激素水平的下降。我们还研究了是否哺乳期暴露于2,3,7,8-四氯二苯并-对二恶英（TCDD）是完全负责在新生儿时期甲状腺激素稳态的扰动。妊娠第15天给予妊娠大鼠单次口服1.0 μg TCDD/kg体重。将1 / 2仔鼠与1 / 2仔鼠于产后1天（PND）交叉饲养，分为对照（C/C）、产前仅接触TCDD （T/C）、产后仅接触TCDD （C/T）、产前和产后均接触TCDD (T/T) 4组。在PND 21， C/T和T/T组，而不是T/C和C/C组，显示两性血清总甲状腺素（TT4）和游离甲状腺素（FT4）浓度显著降低，血清促甲状腺激素（TSH）水平显著升高，特别是雄性幼犬。这两组雄性和雌性幼崽肝脏中TCDD的浓度均显著升高，肝脏中CYP1A1 mRNA被显著诱导，CYP1A1免疫染色强烈。在小鼠肝脏中诱导UGT1A6和UGT1A7 mrna，可见UGT1A6的免疫染色。通过在PND 1日暴露于哺乳期的幼崽胃中残留的母乳中检测到TCDD，证实了TCDD从母鼠转移到幼崽身上。目前的结果表明，在哺乳期，而不是在子宫内，暴露于TCDD是负责甲状腺激素稳态的破坏。少

To determine whether the disruption of thyroid hormone and retinoid homeostasis that occurs following exposure to 2,3,7,8-tetrachlodibenzo-p-dioxin (TCDD) can be mediated by the arylhydrocarbon receptor (AhR), pregnant AhR-heterozygous (AhR+/-) mice were administered a single oral dose of 10 μg/kg TCDD,at gestation dray 12.5. Serum and liver were collected on postnatal day 21 from vehicle-treated control or TCDD-treated AhR+/- and AhR-null (AhR-/-) mouse pups. While TCDD exposure resulted in a marked reduction of total thyroxin (TT4) and free T4 (FT4) levels in the serum from AhR+/- mice, TCDD had no effects on AhR-/- mice. Gene expressions of UDP-glucuronosyltransferase (UGT)1A6, cytochrome P450 (CYP) 1A1 and CYPIA2 in the liver were induced markedly by TCDD in AhR+/- but not in AhR-/- mice. Induction of OYP1A1 in response to TCDD was confirmed by immunohistochemical evidence in that CYP1A1 protein was conspicuously localized in the cytoplasm of hepatocytes in the centrilobular region … More . The levels of retinyl palmitate were greatly reduced in the liver of TCDD-exposed AhR+/- mice, but not in vehicle-treated AhR+/- mice. No effects of TCDD on retinoid levels in the liver were found in AhR-/- mice. We conclude that disruption of thyroid hormone and retinoid homeostasis is mediated entirely via AhR. Induction of UGT1A6 is thought to be responsible at least partly for the decreased serum thyroid hormone levels in TCDD-exposed mice.We also studied to clarify whether lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is entirely responsible for the perturbation in thyroid hormone homeostasis during the neonatal period. Pregnant Holtzman rats were given a single oral dose of 1.0 μg TCDD/kg body weight on gestational day 15. Half of the litters were cross-fostered with the half of the dams treated with vehicle on postnatal day (PND) 1 to make four groups of rats, control (C/C), prenatal TCDD exposure only (T/C), postnatal TCDD exposure only (C/T), and both prenatal and postnatal TCDD exposure (T/T). On PND 21, the C/T and T/T groups, but not the T/C and C/C groups, showed a significant decrease in serum total thyroxin (TT4) and free thyroxin (FT4) concentrations in both sexes and a significant increase in serum thyroid-stimulating hormone (TSH) levels, particularly male pups. These two groups of male and female pups had significantly higher concentrations of TCDD in the liver, with marked induction of CYP1A1 mRNA And intense immunostaining of CYP1A1 in the liver. UGT1A6 and UGT1A7 mRNAs were induced in their livers, with marked immunostaining of UGT1A6. The transfer of TCDD from dams to the pups was confirmed by the detection of TCDD in mother's milk remaining in the stomachs of lactationally exposed pups on PND 1. The present results demonstrate that lactational, but not in utero, exposure to TCDD was responsible for the disruption of thyroid hormone homeostasis. Less